To Yerk. v.a. [Of unknown etymology.] To throw out or move with a spring.
A leaping horse is said to yerk, or strike out his hind legs, when he flings and kicks with his whole hind quarters, stretching out the two hinder legs near together, and even, to their full extent. Farrier's Dict.
Their wounded steeds
Fret fetlock deep in gore, and with wild rage
Yerk out their armed heels at their dead masters. Shakesp.