This is useful when you want your puppy to avoid going near something. You can use LEAVE IT when something drops on the floor that you do not want your puppy to touch, when you put food on a low coffee table, or when there is something on the ground outside such as feces or a carcass. Use LEAVE IT anytime you want your puppy to avoid touching something. Do not confuse WAIT with LEAVE IT. WAIT puts the puppy on temporary hold, whereas LEAVE IT means don't go near it unless I tell you to TAKE IT.
To teach your puppy LEAVE IT, place a treat on the floor a little ways away from your pup. If he starts to go toward the treat to take it, cover the treat with your hand or foot. When pup stops attempting to get the treat on the floor from under your foot or hand, reinforce him with a treat from your other hand with the command TAKE IT. Once he gets the idea and doesn't approach the treat on the floor, but instead looks up to you, you can begin to say LEAVE IT right after you toss the treat on the floor. You want the pup to learn that when he hears LEAVE IT, he comes to you for a reward rather than picking up what is on the ground.
Watch your puppy carefully for signs of avoidance when you toss the treat down. Signs of avoidance are different for individual pups. They can be so subtle that if not watched for carefully, they can be missed. Typical signs can be looking away, looking up at you, or moving his head away from the treat. Once he shows any sign of avoidance, immediately praise him and reward with a treat from your hand or other source. You can give him permission to TAKE IT as you do so. When the response is reliable, your puppy should immediately and consistently avoid anything when you say LEAVE IT. This pattern can benefit from being practiced for the lifetime of the dog.
Here’s another way to practice LEAVE IT. Find a safe area to work in that is big enough for you and the dog to move around in with ease. Place a very yummy treat right in the middle of the training area. Make sure it's highly visible to the dog. Put a bunch more very yummy treats in your pocket and go get the dog - on leash.
Walk up towards the treat but not close enough for the dog to get it. Once the dog sees the treat and starts pulling towards it stop moving and do not say a word. Just stand there. The dog will (eventually) look back at you to see why you aren't moving forward. When the dog looks at you, GOOD LEAVE IT, give them one of the treats from your pocket and walk the dog away from the treat on the floor. Put the dog away somewhere, go move the treat that was on the floor to a different location and start again.
Repeat this as often as necessary. You'll know when the dog understands when they begin to look at you as soon as they see the treat on the floor. After all, they are now learning that seeing a treat on the floor and looking back at the human = getting a treat!