What is the meaning of "Shok Al Yarech"?
Rashi citing Targum Yonasan: People on horses, with people on foot. 1
Radak: No one faced him. All fled, and he struck them. One who flees, when he falls on his face, his calf [curls up] on his thigh.
Rashi: Shok refers to one who rides a horse, for he does not support himself on his Yarech (thigh), rather, on his Shok (calf); one foot is in the iron holster that hangs from the saddle. Radak - Yarech refers to one who rides a horse. Lev Yitzchak - Ramban (Vayikra 1:16, b'Sof says that sometimes the Targum reverses the order. (Perhaps here he changed the order and wrote with, to teach that "Al" implies after (first he killed Yarech, i.e. those on horses). "Al Olas ha'Tamid" (Bamidbar 28:10) means after the Tamid - Rashi 28:10 and Sifsei Chachamim, from Sifri Pinchas 144. - PF)
What is "bi'Se'if"?
Rashi: It is the crevice [in the rock of Eitam], like "Se'ifei ha'Selaim" (Yeshayah 57:5).
