For Dravin's dads information as this post is years old.
The brake works on the layshaft which has the effect of locking the diff so each wheel drives in an opposite direction.
This is the same way the P position works in an auto transmission.
So if the brake locks on the mower will turn the wheels in opposite direction when in the air or if the front of the mower is lifted off the ground.
Obviously if the front has weight on it which prevents the mower turning circles around the rear axel you come to a stop.