If your dog is hovering when you throw the dumbbell, you can use opposition reflex.

You can practice with food first.

1. Ask the dog to SIT.
2. Toss a treat and then pull the dog with the leash (not too harshly) forward. The dog will struggle against the pressure, the moment his bottom touches the ground, mark and reinforce.

You can then use a ball to increase drive. It helps to have an assistant to help with the forward pressure, or you can use a Flex Pole. The concept is that the act of throwing becomes a cue to push into the sitting position. This solves hovering