Gemara

53b - Mishnah: The Kohen Gadol sprinkled from the blood (of the Par) once above and seven times below. He did not aim to sprinkle above or below, rather, k'Matzlif.

54b - Question: What does 'Matzlif' mean?

Rav Yehudah gestured like one who lashes.

Beraisa: He does not sprinkle on (top of) the Kapores, rather, against its thickness. When sprinkling above he points his hand down; when sprinkling below he points his hand up.

Question: What is the source of this?

Answer (Rav Acha bar Yakov): It says regarding the goat's blood "V'Hizah Oso Al ha'Kapores v'Lifnei ha'Kapores";

Question: "V'Lifnei ha'Kapores" is superfluous, for this is also said regarding the bull's blood (and we learn from it that also the goat's blood is sprinkled seven times below)!

Answer: This equates the Haza'os Al ha'Kapores to those Lifnei. Just like those Lifnei are not literally on it (rather, they fall to the floor in front of the Aron), also those Al.

Question: We should say oppositely! Al ha'Kapores regarding the bulls' blood is superfluous, for this is also said regarding the goat's blood (and we learn from it that also the bull's blood is sprinkled once above)! Let us say that it equates the Haza'os 'Lifnei' the Kapores to those 'Al' it. Just like those Al are literally on it, also those Lifnei.

Answer: The first way is reasonable. "V'Lifnei ha'Kapores" regarding the goat's blood equates the Haza'os Al ha'Kapores to those Lifnei, and "Al ha'Kapores" regarding the bulls' blood teaches like d'Vei R. Eliezer ben Yakov:

Beraisa - d'Vei R. Eliezer ben Yakov: "El Penei ha'Kapores Kedmah" - this is a Binyan Av (precedent), it teaches that 'Penei' always refers to the east.

But if Al ha'Kapores regarding the bull's blood equates the Haza'os Lifnei to those Al, what do we learn from "Lifnei ha'Kapores" regarding the goats' blood?!

15a: We never find that some of the blood is Nizrak on top and some on bottom.

Question (Mishnah): He sprinkles once above and seven times below.

Answer: He was k'Matzlif.

Question: What does this mean?

Answer: Rav Yehudah gestured like one who lashes.

Rishonim

Rashi (15a DH k'Matzlif): The Gemara thought that one Haza'ah is above half the thickness of the Kapores and seven are below. It answers that rather, they are k'Matzlif.

Question: The very Mishnah from which the Gemara asks (53b) explicitly says that he did not aim to sprinkle above or below, rather, k'Matzlif!

Answer (Tosfos Yeshanim 15a DH Hizah): The Gemara thought that this means that he does not intend for on top of the Kapores or underneath.

Piskei Rid (Yoma 53b DH Notal): One Haza'ah is above half the thickness of the Kapores and seven are below. However, the blood does not land on the Kapores. Rather, he sprinkles even with it; the blood falls to the ground.

Me'iri (53b DH ha'Mishnah ha'Shniyah): The Haza'ah above should reach the height of the top of the thickness of the Kapores (and fall to the ground). The Haza'os below should reach the height of the bottom of the Kapores. We are not concerned if they went higher or lower than this. K'Matzlif means like one who lashes, i.e. he does not aim for a particular spot.

Rashi (53b DH Ela k'Matzlif and 55a DH ki'Mnagdana): K'Matzlif means in a line, one under the other. One who lashes starts at the shoulder and goes down.

Lechem Mishneh (Avodas Yom Kipur 3:5 DH Achas): According to Rashi, when the Gemara (55a) says 'when sprinkling above he points his hand down; when sprinkling below he points his hand up' does not explain k'Matzlif. Rather, it reconciles the verse "(Al ha'Kapores v'Lifnei ha'Kapores)" with the Mishnah (he did not intend to sprinkle above or below). In Romi, R. Elazar b'Rebbi Yosi saw drops of blood k'Sidran (in a line) on the Paroches (57a). This supports Rashi (Haza'os on the Paroches were just like those between the staves - "V'Chen Ya'aseh l'Ohel Mo'ed.")

Tosfos Yeshanim (58a DH she'Lo): 'K'Sidran' means one above and seven below. They need not be in a line.

Tosfos Yeshanim (15a DH ki'Mnagdana): One who is lashed leans over. A third of the lashes are on his front. The one who lashes whips underhandedly; (when he finishes the stroke) his fingers face up. For the lashes on the back he whips overhand; (when he finishes each stroke) his fingers face down.

Aruch (Erech Matzlif and Erech Nagad): When one hits he lifts his hand but does not strike until lowering his hand to his navel. All the Haza'os were in one place.

Rambam (Hilchos Avodas Yom Kipur 3:5): He sprinkles Dam ha'Par eight times in the Kodesh ha'Kodoshim between the staves of the Aron within a Tefach of the Kapores - "V'Lifnei ha'Kapores Yazeh".

Mayim Chayim (brought in Likutim in Frankel Rambam): Perhaps he learns from "El Penei ha'Kapores". 'Panim' is always at least a Tefach.

Har ha'Moriyah (21): The Rambam means that he sprinkles within a Tefach of the top of the Kapores (even with the thickness of the Kapores).

Rambam (ibid.): He sprinkles once above and seven times below. We have a tradition that "Sheva Pe'amim" is in addition to the first Haza'ah. He does not intend to sprinkle above or below, rather, k'Matzlif.

Tosfos (Zevachim 15a DH Ela): The Haza'os (between the staves) are Kosher even if they did not touch the Kapores.

Har ha'Moriyah (ibid.): The Gemara explicitly says that the Haza'os need not touch the Kapores; he sprinkles on the ground! Rather, Tosfos discusses the Haza'os towards the Paroches. The text of Tosfos should say that the Haza'os are Kosher even if they did not touch the Paroches.

Acharonim

Si'ach Yitzchak (55a DH Adaraba): The Gemara did not merely ask why we prefer to learn Al from Lifnei. It said that it is better to learn Lifnei from Al. This is because it is more reasonable that they resemble Haza'os on the Mizbe'ach, which touch the Mizbe'ach.