The belts in both of my vehicles have to be pulled suddenly in order to lock, but in order to pull it slowly like leaning forward you really do have to actually use a little force lol. If a child fell asleep and leaned a bit I don't really know if it would work, especially if it was hooked onto the positioner on a high back booster. Don't know for sure. If my ODS falls asleep in the high back booster in DH's truck (he is still in a 5 point harness in my car), his head just slumps against the side of the seat and the belt and seat together seem to just kind of hold him there. If it didn't, I wouldn't let him ride it in because kids fall asleep in the car and that's not safe to be all over the place when asleep. I just kind of think either a shoulder belt isn't being used, or he isn't being strapped in properly or in the proper seat. If there's no back to the booster, or no car seat at all, then depending on age and size and whatnot, it's easy for a child to be small enough to slide all over because the shoulder strap wouldn't even be effective.