I don't ever use the strap. I bought the aftermarket rope with a slip knot that summit sells. It used to be standard equipment on the earlier harnesses. I just attach it when I start climbing and move it up as I go. That way you're always attached to the tree.
This may go without saying, but you must tie the bottom of the safety rope to the climbing sticks. Otherwise the rope is slack and you can't slide the knot on your way up.