Nah, it starts to shrink way before you get to the appropriate power. Watch closely and whip your club to 100% and back to 0 a few times fast.. It only shrinks a certain amount and then stops. If you hold the backswing at 100% you'll never see it (forgiveness zone is smallest at 100%).
If you only draw back to 50%, it'll shrink a little as you draw back to 50%. Once you hit 50% and hold it there for a few seconds you'll begin to see the zone gradually start shrinking again down to the size it would had you tried a full shot at 100%.