R. Yonah: Because he is known and established to testify properly, know that also in his conversation and telling matters other than testimony, he does not lie. Had he taught his tongue to speak Sheker, he would not avoid adding to his testimony what he did not hear, or detract from it.

Malbim: He says Emes that is forever Emes - "Sefas Emes Tikon La'ad" (12:19). Hafach refers to incomplete speech. He cannot complete it, lest his Sheker be recognized.

Malbim (according to Melitzah): One who testifies about the laws of Chachmah and Emunah, he is not Mechazev, for his words are always established. The laws of Emunah are eternal Emes.

Rashi: He constantly speaks Sheker - a false witness.

R. Yonah: One who testifies falsely, all his conversation and telling matters are established to be Sheker. His habit to lie in conversation caused false testimony to be light in his eyes. Yafi'ach applies to constant conversation, It is the lips' blowing of breath. Above (12:17), it says "Yafi'ach Emunah Yagid Tzedek." One whose entire conversation and telling matters is Emunah, you know that when he testifies about his colleague, hatred or love of bribes will not tilt him, since he is used to choosing Emes in all his matters. "V'Ed Shekarim Mirmah" (ibid.) - one who accustomed himself to Mirmah, do not rely on his testimony. It is light in his eyes to testify Sheker about his colleague.

Malbim: Sheker is immediately openly false. Kezev initially appears Emes, and at the end it is found to be Sheker. It is something that ceases and does not last. An Ed Sheker says Kezev. He will not say what is openly false, rather, what is later found to be false.

Malbim (according to Melitzah): An Evvil, who testifies Sheker to argue with the laws of Chachmah via his doubts, he spews Kezev and misleading comparisons. After refinement and testing, it does not last.