charles wrote:What I see in that video is mostly rotation at the ankles accompanied by a weight shift. If that is what is meant by "rotating the crotch", then, as I've learned it, it is undesirable. If you eliminate the weight shift to isolate the primary action, you'll see that mostly he is simply rotating the entire body - as a "slab" - from the ankles. (Try it.)
Another way to view that is turning from the hips and that is a correction/admonition you see in Chen taiji - do not turn/twist the dang (crotch). The character niu is used which means turn/twist and also swing the hips. A basic idea of body mechanics in taiji is that the waist is like an axle. If you turn the hips, not the waist, you'll get that correction, but I can't really say if that is what the OP means without context.