Can't argue with Bas. He hits harder than anyone on here, and he throws his hooks with his elbow slightly below his hand. That said, these are hooks with a slightly upward vector...so the elbow is still "behind" the punch. He does this to be able to protect is sides better. Feet are flat, and the torsion in the waist is providing the power. Punch isn't as tight as the muay thai or boxing hook.
The MT hook, is one that starts with elbows down, comes up to target (where necessary) and turns horizontally as the punch moves across the body's centerline in a c-shape, with the arm coming back quickly to protect the side once more. Weight on balls of feet with front foot flat and rear heel lifted, torsion comes from the waist. Tight punch.
The standard lead boxing hook, the way I learned it, was a parallel to the ground vector with the arm coming in in a "tight c" shape. The front foot is on the ball of the foot with an inward turn and the rear foot is flat. Also should be a very tight punch.