What causes heat/air to come from the floor and defrost and not the vents?

Generally, there are flaps that direct the air. Depending on the vehicle, they can work in *slightly* different ways. I can specify with my car where I want the air to go. My taurus would only put heat out the vents if I moved the controls to AC. You can use AC, just adjust the temperature to hot. It works the same, just changes the temp. You can hear the flaps move when you change how you want it to go. My Taurus had a flap that would get "stuck" and I would have to tap on the center console with my feet to get it to adjust.