Then with a gundog you might blow the stop whistle and dog sits. It doesn't move as it's waiting for a command. Then throw a ball to its right and it stays until a hand signal is given for it to fetch. The dog has already mastered steadiness to the fall at this point. In this way stopping to the whistle becomes fun because dog relates stopping to something fun but of course its still under control.
Then gradually increase the distance between you and dog when you blow that whistle. Further away the dog is the more likely he is to ignore you.
You can sit them. Walk away. Call your dog to you and when they are fairly close blow the stop whistle. They sit you walk to them and praise. Then gradually blow the whistle when the dog is further from you. Mine quickly learn this so don't do it too much as they start coming back slowly waiting for the whistle! Ha!
With gundogs the stop whistle is used when the dog is looking for an unseen dummy/bird and might need help finding it from its handler. It stops and looks to the handler who gives a hand direction. You don't really want them to sit as they get to know the lesson just to stop quickly and look to you for a command.
Now, what do you do if your dog has gone through the many steps of teaching the stop whistle, understands it, has shown that they understand it but chooses to ignore it?
?? I've never known a dog that wouldn't choose on occasion to ignore it eg. Mmmmm, stop or chase the rabbit, sooner run after that dog and play.
This is when I might gently reprimand a dog while in the act of ignoring that stop whistle. In the long term I see it as a positive. Dog can enjoy retrieving that wouldn't be possible if it wasn't trained to that whistle and for safety it knows not to ignore that whistle.