As a horse rider myself, I can just say that there's a lot of thrusting going on when you're on a horse, as well as you build ridiculous muscles and core strenght. What I think Jack is referring to when he says that is that Ennis can really f*** him silly, because he knows goddamn well how to thrust
just right, he can do it hard, fast and for a loooong time. He's got that core strenght, he's got the pelvis control and he for sure has the stamina to not wear out easily.
People who has never been on a horse don't seem to understand how physically challenging it is to ride, and considering that Ennis rides western, which is both hard and fast in comparison to english riding, he probably built up even more of that strenght and stamina.
Tldr; Jack is complimenting Ennis for how good he is at !@#$.
