Release leakage is when I feel that I haven't made the effort to ensure the grip completon has been achieved and this results in a leakage of leverage on the ball as the wrists might start to come into the swing.
As for the exercise with the rubber strap, it is a wide strip of rubber which I attach to a door handle. I adopt the Mindy stance and then pull the strap over the ball position. I get the feeling that this strengthens he very muscles I need to propel the ball. I'll get back to the Forum with results.