I have difficulty getting students to understand the “pullback hand” principle, in part because it is badly named. The issue has nothing to do with your hand.
Think of the gasoline gauge in your car. We like seeing the needle on “F” and we get nervous when it points to “E”. So there it is, wobbling over the “E”. We want it back on the “F”. Can we fix the situation by reaching inside the gauge and pushing the needle from “E” to “F”? Clearly not.
Picture a student doing oi-zuki in zenkutsu dachi. His punching hand is straight out in front, but his pullback hand is too far forward and looks generally loose and unconnected. So we move his pullback hand to his hip and tell him to keep it there! That’s the same thing as shoving the gas gauge needle from “E” to “F”. It does no good at all.
The principle of hiki te is simply that you are supposed to clench your back muscles when you focus a hand technique. (There’s a nice picture of Funakoshi somewhere, clenching his back muscles for the camera. They stood out in knots.) When your back muscles are tightly clenched, the pullback hand points at your hip, just like the needle pointing at “F”. If the hand is anywhere else, it means that the back is relaxed. The student should be reminded to clench his back muscles, not told to reposition his hand!
The back contraction is needed in order to stabilize the punching shoulder for impact. The shoulder is a “floating” joint with no bone-to-bone connection to the torso... a natural shock absorber that robs us of impact unless we lock it down with tightly-contracted sets of opposing muscles.
It is said that there is also the dynamic side of hiki te, in which the power of the hand pulling back lends some force to the punching hand. I no longer believe that this is true.
Hiki-te is an element of internal focus.