One great thing about the vest is that it positions the seat belt to the child even when the child moves out of correct seating position in the vehicle. That being said, it would be safest if the child was sitting up correctly.
To accomplish this you can lock the seat belt. With most seat belts you do this by pulling it all the way out and letting it go back in. While it goes in you’ll hear a quiet clicking and when you pull it it won’t come back out, then you know it’s locked.
You also can use the top tether to help support the child in sitting correctly. Some children find it more comfortable to sit “criss-cross-applesauce” with the vest on.
We also recommend another product called the Cardiff Headrest. It’s a great companion to the vest for those children (or long rides) who fall asleep and could use a place to rest their head. You can find Cardiff Headrest on our store here.