yeah, it means rest-pause. so do the 15 reps, take 4-5 breaths, do 5 more, repeat. each mini-set should be challenging for the scheme to be effective, borge fagerli recommends going to within one rep of failure on the first set and then doing mini-sets until you can't hit the target number of reps anymore. that's how i usually do it, just decided to shoot for 30 total reps today instead. not sure why. it's a way to get a decent amount of moderately heavy reps in, quickly.