Ask your dog to sit. Hold a high-value treat in your hand and let them sniff it.
Slowly lower your hand with the treat straight down between their paws. Their head should follow the treat.
The moment their elbows touch the floor, mark the behavior ('Yes!' or click) and give them the treat and praise.
When your dog is reliably following the lure into the down position, start saying 'Down' just before you begin lowering your hand.
Gradually start using an empty hand for the lure motion, rewarding after they lie down. Over time, make the hand signal smaller.
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