The "brake" function in an ESC is to brake the motor,..i.e. stop the prop. A windmilling prop (no power from motor and no brake) is the highest amount of drag you will find. Using the ESC brake function to stop or slow the prop will decrease drag substantially.

It's very easy to see the drag relationship between a stopped, windmilling, or folded prop if you fly a hotliner a bit. A "true" hotliner,.. with appropriate power, will climb vertically in the 70 to 120 mph range (depends on how much power you shove in the nose ), then with the prop stopped and folded, can get into the 150 to 200 range after a short dive power off. However, if the brake gets disabled on the ESC so the prop doesn't stop and fold,.. and it's left windmilling, it's like flying with a boat anchor attached,.. 20 to 30 degrees nose-down just to keep from stalling. It's VERY OBVIOUS the drag produced from the windmilling prop. On certain funfly planes, i've bolted on folding props just for things like climb and glide, so the prop will stop and fold. That's made the difference in a 2 minute glide after a 20 second climb,.. up to a 7 minute glide after the same climb. The point is,.. if you want aerodynamic braking, you want the prop windmilling freely, not stopped or slowed.