I do this. I ask for obedience, give the command to wait, throw the ball, release Hunter, and command a "platz" half way to the ball. He has very high ball/toy drive so this type of proofing gives me the confidence to have him off leash.
At first, the second he dropped I would release him again to take the ball - his reward for dropping immediately. Over time I added time and made him wait a few minutes before he could pursue the ball. Sometimes I ask for him to return to me, give me focused heeling, then release him to the ball afterwards.
This paid off for me. He can be far away from me and will drop on command. He has done so several times when it was necessary. A huge group of people on a trail on bikes, had him drop to the side and wait for me. Chasing a bunny towards a street while playing in a field, had him drop and then come back to me. It pays off.
Not directed at your dog, but same exercise. If the dog has a tendency to break the down in this case, you can also walk up to the dog into a basic position and either release from there, or sit them and then release.
Hunter, CWDC UScA Helper
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