True if you're doing it incorrectly
Firefighter training here
Put your knee or both knees into the car seat then buckle the belt
Now pull the belt all the way out to activate the self-locking mechanism - almost EVERYONE misses this step
Still holding the excess, make a loop of webbing on the side with the buckle, and use this loop to yank all the slack out of the lap portion while you're putting slightly more weight on the opposite side - to keep the seat from sinking to the buckle side
Feed excess back into the shoulder clickity clackity thing
This entire time, your weight has been in the seat - once you get off - the lack of weight will further pin the car seat into the cushion
Everyone one of my "belt" installs doesn't move at all in any direction
FYI - you should still use the top tether with a seat belt unless your car seat says no
ha don't make fun of me I went to the fire station and had my seats installed (F250) I am so paranoid I will do something wrong and hurt my kids. I donated $40 to the station also
BUT no way not even close on the movement part. my seats now in my Raptor do not move a hair in the F250 the def moved and it was sideways. that don't mean either is safer but to me it is and that's all that matters unless someone told me the seat being solid is bad I will keep it that way