Thats what I meant.The core being stretched backward then forward creates torque. Along with momentum and shift creates leverage for this torque. This leverage rotates the hips. All are needed.
Momentum and shift by itself can create ‘active’ hips (unbalanced ) ( low adjustability).
Core torque by itself can create an ‘around’ swing. Among other issues.
The core being stretched both ways while the COG moves to its new ‘balanced’ position creates a powerful, adjustable, time friendly swing with minimal effort.
sequential resistance along with shift/momentum rotates the hips. ‘reactively’.
There is a "stretch"between rear leg and hip. But there is also many core muscles stretching. The whole torso works together to create suddenness. Throw bats. Launch bats as far as you can and you will develop a natural sequence to produce power.
The rear leg is the Driver.
My concern with saying the torso turns and the hips are along for the ride is that the lower half never gets engaged.
The question was ‘what causes the hips to rotate in a HIGH LEVEL swing? Not what works for you? And not ‘how can I get a kid to rotate fully’.
Actually in some HIGH level swings the hips don’t fully rotate. But the core does. Go figure.