Teaching a snappy down: "Down"
I always tell people that sit and down don’t even make it onto my list of foundation behaviors - but I’m lying - when you’ve got the rest of this stuff on the list under control, it is helpful to teach your puppy to sit and lie down :)
These videos focus on teaching the down which often is more challenging than the sit.
You can play similar games to make a nice fast sit something your dog is happy to do as well.
Notice that at first I reward mostly in the down, and as my dog understands the behavior, I start to play more and more games where the reward comes because of the down, but not in position. You do need to keep rewarding in position at least some of the time, but this enables you to reward down (or sit) with a ball throw, or with a chance to jump in the car, or with a game of tug - without being constrained by only being able to pop a cookie in the dog’s mouth as they are in position as a reward.