"Leave It" Cue
Dogs will eat anything! They are faster than us, too. Leave It can save their life if you drop your medicine on the floor or spill a box of chocolates. It is also useful for cats, other dogs, squirrels, people on bicycles or skateboards, etc.
Leave it means 'forget about it'. For it to be effective, we can't tell our dog to leave something and then tell them ok, get it. They will remain focused on it, waiting on GO. Definitely not what we want if it's poison. Use "Wait" for 'pause until I say so' for treats and food. Use "Leave It" when you want them to remove all focus.
A hand signal generally isn't used for Leave It because it's most often said in a panic after you dropped something! But, a closed fist is a good option if you want to use one.
Be careful not to toss the treat too far away. It should land no more than a few inches away from Fluffy's reach.
Practice with safe objects as well. Toilet paper roll cobs, hairspray lids, tupperward lids, etc. When out for a walk, practice with cats, squirrels, etc.