My almost 5 year old son has a booster with a back that we use with the normal seat belt in the car. It feeds through a slot near his shoulder so it doesn't cross his neck and seems to be correctly positioned on his body. We do NOT attach the seat to the car (like we did when using the harness), just put it down, them him in it, and buckle the big seat belt over him.
So the other day my husband had to very minorly slam on the breaks and my son slid forward and said "Haha! My nose touched Daddy's seat!" Is it just that we didn't stop fast enough for the seatbelt to lock? I don't know if his car seat scooched forward or just his body.,. we are not supposed to buckle the seat in with the latch system, right?