I stop them if at all possible, aloud, high pitchedwhistle has always worked best for me. Sometimes you have no choice, I have dropped deer on the move when I had to, but I don't like it. It's not very precise. I killed an eight point a few weeks ago at a full bore run, but I had spooked him and it was a last ditch effort, I knew a whistle wouldn't stop him. You just have to make the call depending on the situation