I call this the endurance sit because it buids up endurance as well as muscle strength in your quadriceps.
It may help to think of it as “sitting on air,” because that's what you're trying to do. Put as much of your weight as you can back, wighout falling down. It really is like sitting down: try to put as much weight as you can onto your “chair” without falling over. Of course, you won't be able to put all of your weight onto the “chair,” but you should do enough so that you feel that your quads are supporting you. Trust me, you'll know when they are. Check out this article that defines what bodyweight exercise is.

Also, there are 7 Great Reasons Why You Should Do Bodyweight Exercises.