Gemara

(Mishnah): If a Kohen was standing on top of Kelim or an animal or another person's feet and he did Kemitzah, it is Pasul.

Zevachim 24a (Tana d'Vei R. Yishmael): Just like a Chatzitzah between the Kohen and the Klei Shares disqualifies, also between him and the floor.

Had the Mishnah taught only about standing on top of Kelim, one might have thought that this is Chotzetz because Kelim are not flesh, but an animal is not a Chatzitzah;

Had the Mishnah taught only about standing on animals, one might have thought that this is Chotzetz, but standing on a person is not.

110a (Mishnah): If one offered Kodshim with the Eimurim attached, he is liable.

Question: Why is he liable? The meat is a Chatzitzah (between the Eimurim and the wood on which he burns them!)

Answer (Rav): Min b'Mino (the same species, i.e. the Eimurim are also meat) is not a Chatzitzah.

Yoma 36a (Beraisa): To do Semichah, one puts both his hands between the animal's horns. Nothing may interrupt between his hands and the Korban.

58a - Version #1 - Question (Rami bar Chama): If one put one bucket inside another, and collected the blood in it, what is the law? Is Min b'Mino a Chatzitzah?

Answer (Mishnah): If he was standing on top of Kelim or an animal or another person's feet, his Avodah is Pasul.

Rejection: A foot is different, for one cannot be Mevatel it (permanently leave it there).

Version #2 - Question (Rami bar Chama): Is holding a bucket with a bucket inside a proper way to serve?

Shabbos 120b (Beraisa): If Hash-m's name was written on a person, and he has a Mitzvah to immerse, he wraps a reed around it and immerses;

R. Yosi says, he immerses normally, as long as he does not rub it.

(Rabah bar Shila): Chachamim forbid Tevilah (immersing) because one may not be naked in front of Hash-m's name.

R. Yosi holds that he covers the name with his hand. Chachamim forbid, lest he forget and remove his hand.

Objection: R. Yosi should also be concerned for this! Rather, all agree that if a reed is available, he must use it. They argue about whether or not he must toil to find a reed:

Menachos 94a: If many partners would wave a Korban at the same time, not everyone could touch the Korban, for others' hands would be in the way. This is a Chatzitzah!

Rishonim

Rambam (Hilchos Bi'as Makdish 5:17): One who serves in the Mikdash must stand on the floor. If there was a Chatzitzah between him and the floor, e.g. Kelim or an animal or another's foot, this disqualifies.

Rambam (Hilchos Ma'aseh ha'Korbanos 9:15): If two brought a Shelamim jointly, one waves with the other's permission. Even if 100 brought it, one waves on behalf of all of them.

Rambam (19:5): One who offers an entire animal outside the Mikdash is liable. Even though he did not separate the Eimurim, the meat of the Korban is not a Chatzitzah. It is as if he burned the Eimurim by themselves.

Rosh (Berachos 3:36): When R. Tam would bathe in hot, clear water and he wanted to drink, he would cover below his heart with a garment when blessing, so his heart would not see his Ervah.

Terumas ha'Deshen (10): We are more lenient about covering Hash-m's name during Tevilah than regarding covering the head for Berachos. If not for concern lest he forget, we would permit relying on his hand of Tevilah. For Berachos, a Gadol brought from Or Zaru'a that covering the head with the hand does not help. He learns from R. Tam, who used a garment, and not his hands. However, perhaps Berachah are not more stringent. The hand does not help to cover the head, or to separate between his heart and Ervah, because they are all one body. The body cannot cover itself. Regarding Hash-m's name, the hand covers a name. It need not cover the body.

Tosfos (110a DH Min): Above we say that Avodah while standing on another's foot is Pasul. This is because it is not the way to serve.

Tosfos Yeshanim (Yoma 36a DH u'Bilad she'Lo): Toras Kohanim expounds "v'Samach Yado Al Rosho", and not one hand on the other. All the more so something else is Chotzetz!

Poskim

Shulchan Aruch (OC 74:2): One who is bathing and wants to drink must use a garment below his heart to separate it from his Ervah while blessing. The hands are not a cover.

Magen Avraham (3): For Tevilah the hand is a good cover (for Hash-m's name). Here is different, for it is one Min, and Min b'Mino is not a Chatzitzah. Also, (even for Tevilah) we decree lest he remove his hand!

Rema: Likewise, covering one's head with his hands is not considered covering.

Taz (2): It is not a covering to bless or say words of Kedushah, but it suffices to avoid sitting or walking with an exposed head. For this, the hand helps.

Kaf ha'Chayim (12): Some people are meticulous not to walk with an uncovered head even in the bathhouse. They can cover their heads with their hands.

Be'er Heitev (2, citing Pri Chodosh 2): If Reuven puts his hands over Shimon's heart, this is a valid cover.

Mishnah Berurah (9): The hands do not help for a cover or interruption, unless he clamps his arms.

Shulchan Aruch (3): If one clamps his arms (over his chest), this is an interruption.

Beis Yosef (DH v'Chosav): R. Yonah says in the name of Chachamim of France that if one clamps his arms on his body, this is like interrupting with a garment. The Terumas ha'Deshen holds that covering the heart with the hand is not a cover because it is one body, but perhaps he agrees that the hand can interrupt, even though it is part of the same body.

Mishnah Berurah (14): It helps because he does an abnormal action. The Pri Chodosh and Maharam Tivlu disagree, but even they agree that if one wears a robe without pants, it helps to press it over his heart or against his body below his heart.

Shulchan Aruch (91:4): Putting one's hand on his head is not considered covering. If Reuven puts his hand on Shimon's head, it seems that this is considered covering.

Beis Yosef (DH v'Chasuv): Terumas ha'Deshen says that the body cannot cover itself. This connotes that if Reuven puts his hand on Shimon's head, this helps.

Birkei Yosef (74:5): We can bring somewhat of a proof for this from Zevachim 110. Rav answered that Min b'Mino is not a Chatzitzah. Tosfos said that Avodah on another's foot is Pasul because it is not the way of serving. In Menachos 94a we say that if partners would wave a Korban at the same time, others' hands would be a Chatzitzah. Based on Zevachim 110 and Tosfos, Min b'Mino is not a Chatzitzah! In Zevachim 24a, we say that standing on top of another's foot disqualifies due to Chatzitzah. Also Yoma 58a connotes like this, unlike Tosfos. Tosfos' question was not difficult. In Yoma we answered that a foot is different, for one cannot be Mevatel it! It is difficult to say that Tosfos relied on the other version in Yoma. If he could be Mevatel the foot until the Avodah was finished, it would not be a Chatzitzah. Why does Menachos 94 say that partners' hands are Chatzitzos? Min b'Mino is not a Chatzitzah, and there they are the same (i.e. they are Mevatel their hands - PF) until they do Tenufah!

Birkei Yosef: Why does Menachos 94 say that partners' hands are Chatzitzos? Min b'Mino is not a Chatzitzah, and there they are the same until they do Tenufah! The Bartenura explains that the other's foot is a Chatzitzah, that meat is not a Chatzitzah because Min b'Mino is not a Chatzitzah, and that partners cannot wave together due to Chatzitzah. The Rambam rules that a Chazakah between the Kohen and the floor, e.g. Kelim or an animal or another's foot, disqualifies. He says that one partner waves for all of them, and that one is liable for offering an entire animal outside the Mikdash, because the meat of the Korban is not a Chatzitzah. How can we resolve these? Rav said that meat of the Korban is not a Chatzitzah regarding Eimurim of that same animal, since they are one body. It is totally Min b'Mino from one body. We do not say Min b'Mino is not a Chatzitzah regarding another's hand or foot, even though they are both flesh. Since they are two bodies, it is a Chatzitzah. This is why the Rambam discusses offering an entire animal. He did not say 'meat and Eimurim', like the Mishnah (even though the Rambam often cites the Mishnah or Gemara verbatim) to teach that this is only when they are from one animal. In Yoma we asked about two Kelim, i.e. two bodies. We could not bring a proof from the Mishnah of one animal. We tried to bring a proof from Avodah while standing on another's foot, i.e. two bodies. Kol ha'Ramaz (Zevachim 13) says that the Mishnah said 'Zevachim' (plural) to teach that meat is not a Chatzitzah even for Eimurim of another Korban. This is like Tosfos, unlike the Rambam and Bartenura.

Question (Ha'aros on Birkei Yosef, 4): The Rambam (Hilchos Pesulei ha'Mukdashim 1:2rules that if one did Kabalah in a Kli inside a Kli, it is not a Chatzitzah, even though they are two bodies!

Magen Avraham (4): The Maharshal (Teshuvah 72) permits (relying on one's own hand). I say that one should not rely on this.

Kaf ha'Chayim (29): Eliyahu Rabah (5) says that in pressed circumstances, one may rely on Maharshal. I say that the Shulchan Aruch and Acharonim said Stam that it does not help, i.e. even in pressed circumstances.

Mishnah Berurah (10): The Bach says that people stretch the sleeve over the hand and put it on the head, and this is a proper covering.

Afarkasta d'Anya (2 YD 136): Min b'Mino regarding Isurim is different than Min b'Mino regarding Kodshim. This is whether one holds that Min b'Mino depends on taste, or on the name. Regarding Kodshim, even a slight semblance suffices to say that it is b'Mino and it is not a Chatzitzah. Had the Mishnah taught only about standing on top of Kelim, one might have thought that an animal is not a Chatzitzah, for it is flesh. Surely, a man and an animal is Min b'Eino Mino regarding taste and name! Rather, even though for Isurim it is b'Eino Mino, regarding Kodshim a mere semblance suffices. In the conclusion, the Pesul is not due to Chatzitzah, so we can say that the Hava Amina was true, that even an animal is not a Chatzitzah. Tosfos is difficult. In one version in Yoma 58a, we asked whether a Kli inside a Kli is not the way to serve (and we answered that it is). No one said so about standing on another's foot!