using words to describe posture on a bike has its limitations. I'm no bike fitter, but when I set up my bike, I work out the saddle position relative to the pedals, then I put my handle bars where I can reach them naturally in a riding posture. There's some degree of tilting the pelvis to engage the sit bones on the saddle, and there's some engagement of core muscles to hold up the upper body, and the degree of forward lean is achieved from the pelvis rotation, not the hunching of the back and sholdures. think of an athletic crouch. put a bike under it, and you'll get the idea. shoulders are generally relaxed. as the core strengthens, the crouch gets lower. there's hardly any stretch in the back going on.