for me the stop whistle starts with the hand signal when you feed the pup command word raised hand flat palm and a food bowl in the other hand, soon as pup looks up at the food/hand walk in a bit and it should park its bum . later on combine this signal with a whistle when out with the pup just one time a session, but don't do it when feeding.I only use same whistle or command for sit- stay- stop,