[Bigwill]

I have a student, who's buddies tell him to pull from the
top with the left arm. I tell him that this is incorrect, that
the left arm is inert with not much power. I recommend that
he start the downswing with shift/pivot/hip rotation. The
student says that he can't move the downswing with the
shift/hip turn, says it hurts his right hip. He insists that the
pull with the left arm makes him pivot.

How do I explain to the student that pulling with the left arm
does not make the pivot happen?

The thing is that, for him, that thought may work perfectly. You have to remember that what the student is feeling, or whatever his trigger is, may not be what is ultimately happening. You also have to take into account his physical problem; if focusing on the hips hurts, then he probably shouldn't use that thought, at least until that problem can be addressed. Otherwise, his body will compensate. In his case, the THOUGHT of pulling with the left arm may be the thought he needs in order to get his pivot to pull the left arm properly. I've seen video of myself when my only thought was to pull with the left arm, and the actual first move was of the shift and turn of the hips.

The point is that, for some people, the pivot does respond to the intent to make a motion (don't make the mistake of thinking that "respond" in this instance means "to trail" or "come after"). For others, if they don't consciously engage the pivot first, then they'll have problems. You have to separate intent from what actually happens. If someone hits the ball well, with the proper sequencing of feet, knees, hips, trunk, shoulders, arms, hands, and club, then whatever they are focusing on (be it hips or arms/hands/club), is right for them.

Whatever the mental key, whether or not the swing is hand controlled or pivot controlled, a good and powerful swing is generally going to actually start from the ground up.

For this student, make sure that this thought is actually hurting his swing, before changing it. Whatever trigger he uses, whatever feel he cultivates, is correct if it results in the correct action.

Let mechanics produce feel, and let feel reproduce, you know?