Counterbalancing.
Different segments of the body can bend forward, backward and sideways, but with the right combinations of contractions, the body can balance itself out to produce propulsion and stability.
The equivalent of firing a canon from a battleship, not a canoe. Here are the 7 core exercises:
Think:
hollow body + lower body pushing
Think:
hollow body + upper body pulling
Think:
standing tall + push against ribs
note: vertical push for swimming
Think:
Standing Tall + Pushing against ribs
Think:
Freestyle: Open with Hips, Close with Torso
Think:
Backstroke: One side bends, opposite side reaches forward