This exercise is to tell your puppy/dog to leave whatever has attracted its attention, a food scrap in the street for instance, to pause and wait for you to tell your puppy/dog what to do next, such as, “come”, or “get on”. (This is how to teach the early stages of this exercise).
Start with 2 treats in each hand, low value boring ones in your left and high value super jackpot treats in your right. Your puppy/dog should be in a sit, right in front of you, so that it is looking towards you, not across you.
Put your right hand behind your back, out of reach and then put your left hand in front of your puppy’s/dog’s mouth/nose, close enough that it can smell the treat in your hand without needing to get up out of the sit. Your puppy/dog will sniff at your hand, perhaps lick it or maybe paw at it, ignore everything that it does in an attempt to get the food. As soon as it takes its mouth away from the food or takes is attention away from it by looking away or at you, immediately treat from your right hand and say, “good boy/girl”. Your reaction needs to super quick, don’t make your puppy/dog wait for your right hand to appear.
After a few tries at this, introduce the phrase, “leave it”, as you extend your left had to your puppy’s/dog’s mouth/nose. Wait for your puppy/dog to change its point of attention, as described above and instantly reward and praise.
Practice this until you can say “leave it”, put your left hand out to your puppy/dog with a treat and your puppy/dog will completely ignore it, looking for the reward from your right hand instead.
Stage 4; progressing this exercise.
When your dog will consistently ignore your left hand in preference to a reward from your right, try this;
Do the leave it exercise 2 or 3 times so that your puppy/dog is reminded of what is expected of it, then show your puppy/dog the treat in your left hand, saying leave it at the time, then drop the treat to your left, slightly behind you, being ready to step across your puppy/dog if it moves to get that treat. (If this happens, go back to stage 2).
If your puppy/dog doesn’t move, give the reward in your right hand instantly together with “good boy/girl, yes well done”. You must give the, “leave it” cue clearly and firmly before you throw the treat. It is too much to expect that your puppy/dog not to move, at this stage it is likely that your puppy/dog will try anything to get the treat, so be prepared to step across your puppy/dog or to protect the treat with your foot if your puppy/dog goes around you in an effort to get to it. It must not get that treat! Watch for your puppy/dog showing the slightest hesitation and immediately reward that moment, that lack of movement. Do not wait for ages to see how long your puppy/dog will remain in a static position; by doing this you are setting your puppy/dog up to fail, which is entirely not the point of training. Practice until your puppy/dog will just sit in front of you, waiting to be rewarded for not moving.