You legs, should be stuck at that girth 100% of the time over fences. On approach, over, landing, depart.
You are not only aiding your form and solidity in your tack, but you are also aiding your horse and are remaining supportive as you should be.
You are NEVER ontop of your horse. You area ALWAYS around your horse. This is a team effort, if you don't do your job properly, how do you expect your horse to do his/hers? If you are out, how do you expect your horse not to be?