Now that the shooter’s relaxed,pull back slightly from full extension. Have them pull back towards the holster position, noting where the index finger engages the trigger guard. From this low ready position, have them re-extend (as best shown in the video) so they will re-establish their grip. Note that when they first contact that upper portion of their index finger they can roll into a very consistent grip. A common deficiency, of course, is slapping the hand on very late which is extremely hard to consistently place a support hand thereon.