Quote:it FEELS like my body responds much quicker again. Just an observation.

I've heard people say that this actually has a basis in physiology. What happens is that when you switch up a routine, you devote your attention differently to different body parts, which can lead to glycogen store growth (sarcoplasmic hypertrophy) in previously neglected areas. But after a while, the body adapts to the stress, and the initial gains fall off.