Your seat from the mid-thigh up is fine. You sit relaxed. BUT your entire leg below that is useless. Well, not exactly useless because you keep it on the horse and it doesn't flop around. But this is actually the most important part of keeping a balanced seat at any gait on any horse. Your weight should not be sitting on your butt on the saddle, but instead should be down through your leg.
If your horse bolted or spooked you would topple off. It is not the heels down that is the issue but the fact that your weight is not down through the leg. If you had a big ketchup packet on the saddle you should not be sitting down hard enough to break it open.