just read teh new chapter (mangahelpers) and in it luffy uses Gear 3 at the very end and explains it so its basically pumping air into his bones while Gear 2 was into his blood
that explains him getting smaller i guess after peforming the move his body shrinks cuz his bones get smaller and he probably has to slowly regrow it by putting air in it bit by bit
wonder what gear 4 will be
