As you approach the ball bring the racquet under and behind the ball, explode up with your knees and brush straight up. The lower the racquet is under the ball when you begin your swing at it, the more kick. Also bring your toss left.
One caveat to the above mentioned correct answer. Yes switching to the Eastern BH grip will get you more spin and hence more consistency. But it is common when going for maximum spin to shank or frame the ball on occasion. Which is fine on a first serve but you may freak when it's on your second.
So he did not switch the grip but still got more kick. So there must be a biomechanical reason....
You must mantain shoulder rotation, internal rotation of the upper arm, and forearm pronation to achieve racket head speed, which in turn, if the racket is put on the correct path for a topspin serve, will produce lots of spin and speed Changing the grip and the toss will not in themselves produce racket head speed.
Blocking the chest is the best way to get the racket to swing on a more low to high plane to produce spin. Hope that helps.