Yes, the belt should skip a little under acceleration. Mine is set set from the rostrum I cannot hear it or feel it, people have said "I can hear you belt slipping about 2 teeth as you put the power on where I marshal" but that is perfect.
If it is slipping too much, its that familiar racket, not a "click..... click" then silence. i.e. if its slipping to much the clicks can't be counted.
Under braking, yes it should slip and it can sound excessive. The nature of the drivetrain means the belt slackens around the main pulley which makes it slip. You can help this by making a roller out of a bearing which runs on the belt at the pulley - doesn;t press, just holds the teeth in. It'll still skip but about 50% less.
All I do is turn my brakes on my tranny down to about 60-75%, you'll find you don't need any more than this while racing at full tilt and the belt will be miles better. Just be careful of your pitmans ankles...
