This allows the lower back and hips to provide power and weight in playing, instead of using harsh, percussive hand banging. Sitting back creates a terrible inertia which renders any contact with the keyboard tenuous. It is also important to sit far enough away from the keyboard that elbows find some latitude of motion, so that they do not keep bumping into the torso as arms cross the body. One of the worst technical mistakes is to lock the elbow against the body, ensuring rigidity in that joint and limiting mobility for all other connected body parts. Posture also involves how the head is held.
Most pianists suffer terrible tension in the jaws, neck and shoulders.
Although I do not promote Michael Jordan-style tongue hanging, his is a telling case of facial relaxation and what it does for the rest of the body. Sit with the head erect, with the neck extended, instead of emulating the stiff ideal of ramrod straight. Feel that an invisible string is being stretched from the top of the head to the sky. Lifting the sternum orients the breastbone outwards instead of caving it inwards. Keep feet flat on the floor when not using the pedals, or use the heel as an anchor when pedaling.