I think a certain speed threshold has to be met for them to lock up. It isn't much, maybe 5 mph, but I can definitely see a slow speed roll over where the belt didn't lock up, at least initially, at which point the damage might already be done. You can see how much momentum it takes to cause them to lock by just pulling on it quickly vs slowly to see what happens. We have all been driving the road and found out belts needed adjusted, and simply pulled them out to get a little extra slack, so just being in forward motion doesn't make them lock. There needs to be some kind of jolt at a certain speed, I just don't know what speed that is.

This topic has particular interest for me because I have a 4- and 5-year old we take on our trips, riding in the back. Until I put anchors in behind there booster seats they always "walked" there seats forward, even though the were belted in. The anchors help, but a belt with constant tension would give me a lot more comfort about their safety when we are on the trail.