It's what I've heard referred to (and what I've been taught myself) as a "Cookie Cutter" grip. The person who first told me about it taught at MCM and also at an independent open group that I was in, and its basically allowing the hand to stay tension-free by still maintaining a grip on the stick/mallet but allowing the fingers to open to certain angles that facilitate different heights (3", 6", 9", 12", 15"). That's how he's getting a bit more 'outward' motion with his wrist turns, which gives just a tad more velocity behind the mallet for rebound without having to get so much arm motion involved. I was taught it for quads, but similar principles apply to bass since it's also matched grip seeking the same tone from hand to hand.
