καὶ ἀπὸ τῶν ἵππων ἐνεργοὺς αὕτη μάλιστα ἀπεργάζεται
What is the exact meaning of apo here?
I think the basic idea is “from”, as usual - they are on the horses, and from there they are active.
The Cambridge lexicon in section B 2 gives "at (a position or place, fr. which action or movement proceeds); on - a mountain, tower, chariot, horse
So we could say in English “on horseback”, as in the translation, or use an expression like “in the saddle”.