I will point out that when playing and impacting a sub you can hit very hard and not bruise by warming them up correctly, unless bruises are intentional they normally come from too hard hits on unprepared skin
If they are intentional then yes very thuddy hits and not warming up is the way to cause it, but personally i avoid doing that. Bruises prevent me from being used/continuing to use the subby for a few days, and you miss the lovely pain curve by not warming up
(I get it is as a punishment here though)