You can feel the difference just sitting and trying the position. Place your arms in the bench position and squeeze your shoulder blades together. Now perform a mock rep or 2. Feel how tight your back is? Now unsqueeze your blades and widen your back. Again perform some mock reps. Feel the difference? When your shoulder blades are back and tucked it tightens your whole back and core. That combined with a good glute squeeze is the power position.
Berz out.