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Something that might help you get a feel for this position is lowering your bars all the way, so there's no spacers underneath the stem.
Sit on the saddle but DON'T clip your feet in, just let your legs hang freely (easiest on a turbo trainer). To reach the now low bars, you'll probably find you'll have no choice but to rotate at the pelvis, your weight will come off the sit bones slightly, and will end up a bit further forward.
With the "rotated pelvis" position, you'll probably find you need to lower the saddle quite bit to actually pedal smoothly.
Ok. How can I isolate or figure that I'm rotating my pelvis rather than stretching my back?
I appreciate that might sound stupid. Thanks for your patience!