I suspect he's just gotten very used to the instruments he plays, and has it set in his muscle memory how far he has to hold down the bar. Then, it's about how he's holding the bar down, ie with his palm, fingers, or the heel of the hand, and where on the bar he's making contact, the tip, the middle or back at the bridge. From there, it's a question of how he's making the strings work, ie plectrum or finger picking, hammering, or pulling off. If your hand is cramping, try changing where & how you're holding the bar down. If that doesn't help, try changing the way you're picking.
I'm being vague for tow reasons; 1, because not everybody can do things the same way, and 2, I'm not conversient enough with Satriani's technique to tell you exactly how he does it anyway. I'm making educcated guesses here. Maybe there is someone who actually knows what you're asking. In any event, welcome about cool9. By the way, Cool #9 is my favorite Satriani song.
Always remember that you’re unique. Just like everyone else.