Octaves-grasping

when playing light octaves with the grasping motion then the hand muscles works and the by-product is that the wrist moves up and down-why not let the wrist bigger muscles ,which moves it up and down, do the work when playing a lot of octaves in a row and leave the smaller muscles in the hand? -in the vidoes of the octaves the teacher was showing the student how to play p octaves and she wasnt talking about the grasping motion, but rather showing the student just the up and down motion with the fingers,not the grasping motion. thank you