When your childs ears are above the top of the high back car booster seats shell and the child is not yet tall enough or heavy enough to use the cars standard seatbelts then a low back or booster cushion can be used. This is a simple design that consists of a small platform with short arm rests and a method of positioning the lap portion of the cars existing lap & shoulder seat belt. The idea is that the platform raises the height of the child in the car so that the seat belt should strap is positioned across the childs body correctly. The arm rests also help to keep the lap portion of the belt located across the childs hips in the correct way. There are many different brands but the actual functionality from brand to brand will not differ and they all aim to do the same job. Some low back design will come with a strap click to pull down the shoulder strap of the vehicles existing seatbelt to keep it away from the child face. This is fine but if the child is too short that this needs to be used it may be worth considering a high back car booster seat until the child has grown a little.