This isn't necessarily true. To keep your legs moving at the exact same rate as the wheels is close to impossible, and in practice you'll notice that you'll always either be putting pressure forward or backward, but never no pressure at all. If you just relax your legs, the wheels/pedals will carry them, but you have to remember that your legs are very heavy. Relaxing your legs is closer to resisting than coasting.

Say you are riding fixed with clipless pedals and are bombing down a hill. You relax your legs, and your feet are carried by the pedals and you will continue to accelerate. However, there will always be a point at which you can't accelerate anymore because your legs just cannot keep up with the pedals (this threshold is different for everyone). At that point you are resisting and probably bouncing all over the saddle, but you are at your maximum speed. However, if you could coast, you would just continue to accelerate until outside factors, such as the hub, tires, wind, etc., hold you at a maximum.

Anyway I have no idea what this has to do with braking so I'll stop here.