Well Graham, that's exactly what I find myself thinking all the time during bjj rolling from kneeling "now that my legs are missing where is my center" and how can I generate power.
Try focussing on your lower belly between your hips and turning from there. Sort of like leaving your legs and hips where they are and turning your torso between them. Sounds counterintuitive I know. And of course if you turn far enough the legs will follow... Gives a tighter turn and more power.
A simple explanation for something that could become more complex, but maybe this is a start?