What exactly is the La'av of "Lo Sisa Sheima Shav"?
Rashi #1 (citing Targum Onkelos) and Targum Yonasan: It is a prohibition against accepting Lashon ha'Ra (or speaking it Makos 23a). 1
Rashi #2: Judges may not hear a litigant's words before his opponent arrives (and a litigant may not present his claim to a judge before his opponent arrives - Sanhedrin, 7b and Mechilta). 2
Rashbam: It comes to prohibit a. the Dayanim from accepting false testimony. Rather, they must thoroughly cross-examine the witnesses, 3 and b. testifying with false witnesses, even if there are two of them and even without you, their testimony will be accepted.
R. Chaim Paltiel: Do not accept a fraudulant claim.
R. Bechayei: Do not accept a Shevu'ah from someone whom you fear might transgress it.
Pesachim, 118a): "Lo Sisa Sheima Shav": Anyone who speaks Lashon ha'Ra, accepts Lashon ha'Ra or who testifies falsely against his fellow-Jew deserves to be thrown to the dogs.
Oznayim la'Torah: Although Onkelos tends to not differentiate between "Shav" and "Sheker", the word "Shav" generally refers to something that is obviously false
What does the La'av of "Al Tasheis Yadcha ... " entail?
Rashi (according to Divrei David 1 ): Do not promise to testify for someone who claims falsely (even if in the end, you do not testify).
Seforno: It is a prohibition against combining with a Rasha to sign on a document.
Yevamos 25a: It is a prohibition against testifying together with a Rasha.
Sanhedrin 27a: Judges may not accept testimony of a Rasha.
Mechilta #1: It refers to Reuven who says to Shimon 'Levi owes me two hundred Dinrim; and I have one witness. Come and combine with him and you and I will take one Manah each!'.
Mechilta #2: Based on this Pasuk the refined people of Yerushalayim would not participate in a Seudah until they knew who would join them there and would not sign on a Get until they knew who would sign with them.
Ibn Ezra: A witness may not ask someone else to testify falsely together with him.
R. Chaim Paltiel: Do not accept false testimony, and say that the witnesses will bear the punishment.
Bava Kam, 72b: An Eid Zomem is Pasul retroactively (from the time that he testifies)
What is "Lih'yos Eid Chamas" coming to add?
Seforno: In the event that you testify together with a Rasha, it transpires that you are a single [Kosher] witness, and Beis-Din will pervert justice by exacting money from the litigant against whom you are testifying on the basis of your testimony alone.
Mechilta: What is an "Eid Chamas"? If one is asked by one's Rebbe to join a second witness to extract a Manah from the Rebbe's debtor
Why is this written after the verse of Tereifah?