well to comment on resting, when you're doing a lot of physical activity (fighting for instance for dbz) your muscles go through anerobic respiration, meaning exercise that does not use oxygen. the byproduct of anerobic respiration however is lactic acid, where if your muscles build too much lactic acid, thats when it feels like it's going to cramp. also, your muscles will need oxygen in order to do aerobic respiration which requires oxygen. in order for your body to do work quick and efficient oxygen is needed to break down the simple glucose molecule. the more physical activity done (fighting in this case) the more lactic acid is building up and your muscles need more oxygen. thus when resting, your body is not so much in need of oxygen as if it were in physical activity. thus when you rest, you "catch your breath" to gain oxygen to break down the lactic acid to stop the cramps and such.
in this case, goku was very tired and beaten up, and his PL most likely was growing weaker while fighting kid buu. he asked vegeta to fight buu for him so goku can then "catch his breath" and regain himself. thus more energy is not used in fighting and goku can rest to get his body in resting condition, thus allowing him to save more energy and for his body to fix itself for a short while to where he can use his full strength again.
thus i suggested if you rest, you should then have your PL stop dropping because your body should be able to regain itself.
i hope that made sense
