Fast explosive movements on the positive part of rep with slower returns build fast twich muscle fiber that are the fibers that have the most number in the muscle so stimulating those fibers adds more size. Slower returns help avoid injuries as opposed to dropping the weight as fast as lifting it on the positive part and builds slow twich fibers.
So i'm not surprised about your gain on this type of lifting especially if you haven't been doing it before.
If training in this mode you should stop the set as soon the reps start to slow down or use a spotter to keep the speed steady trough the set.