The context is about quick removal of your BP/W harness by another person in water where that person may not know how to handle a BP/W being a BCD user all his life. That user may easily discover that a single tug of the strap on the slider side has loosened it enough to extricate the diver from his gear who is comatose in-water… sure everyone can remove his own BP/W while on the boat with some deft twisting moves of the upper body and shoulder aided by an element of spinal rotation.If your harness is adjusted correctly and you have a crotch strap to keep everything tight, any kind of sliding BS is completely unnecessary.
Personally I like to wear my harness just slightly tighter than a fist-bump because I like my BP/W to rest a little higher up on my back while standing up with the weight of the tank like a well setup backpack. Otherwise the gear not only sags but the wider Zen BP rests on my hip bone (iliac crest I think?) - it may have to do with my ordering a size L Zen BP rather than the S (1” shorter) because my height is in-between the recommendation for transition from S to L.
Meanwhile I am tickling myself trying to picture someone in a BP/W with the recommended fist-bump loose shoulder straps deftly dropping the BP/W to the floor with a flick of her thumb like that sexy woman in a negligee making a move (like in a Hollywood movie)…