I used the blet while I was trying to deal with my SI problem, it did not slip on me, but I kept it pretty tight. I was at the point with this pain that I thought I might have to stop riding, it was that bad. After going to back specialist, chiro’s etc. No one could even really tell me what the problem was except perhaps a hypermobile joint. I finally started accupuncture and then pilates. I have never looked back. I still have a twinge of pain now and then, but nothing else worked. Nothing. Not injections, not chiro, not pain meds. Only the accupuncture and then beginning pilates.
Good luck. It is awful to deal with.