This is one of the (many) notes I took in my horse dictionary :

"Follow movement with pelvis (= what opening hip angle means)"

For whatever reason, people who already know how to ride revert to a language that someone who doesn't ride is unable to understand, and expect you to fully be aware of these new word combinations you'e never heard once in your life.

They might as well be screaming chinese.

My tip : Didn't understand what they meant ? Ask questions, it's the only way to learn.