That's in Wagtendonk and just about every other book, shirley?
Increased rotor RPM because:
Collective is lowered as you try not to climb
Coriolis effect - increased TRT causes coning angles to increase and the C of G of all blades to move inwards
Increased load factor
The lift vector being more forward (because the RAF is shifted downwards) produces an autorotative force and reduces drag
Phil