once you have taught your dog to run to you for the recall, how do you go about training them the down during recall, as it is the opposite of what you were just training?
Actually it's not the opposite. It's just another skill to be taught that is separate from the recall. Again, I like the dog to eagerly want to do it.
So we start to teach it from a front. Have the dog in front of you. Have his favorite motivation present, teasing him a little with it. Say the Down command, and reward instantly and euphorically then release.
Eventually work on the dog maintaining the down position with the reward (if it's not food) in mouth. Once you have an enthusiastic, tail wagging Down then you can start to put in the movement.
You can walk backward (not necessarily in a straight line) and once you have his attention and eagerness to get the reward, command Down and reward again enthusiastically then release.
Once the dog has a strong Down in movement you can start doing it while heeling. You should also start delaying the reward. But continuously praise until you get back to him to give him his reward then release.
Then you build from there. Start the excercise "trialwise". Put him on a Stay go away and Recall him. Give the Down command praising immediately he get into the position. Walk up to him to give him his reward then release.
Now comes the second part of the excercise.... the recall after the Down. Once you reward do not release!!! Take the reward, keep him on the Down and go away to spot where you called him. Once there wait a couple of seconds and when you recall him again, make a small sprint away from him, stop and turn to front him. If you use a toy or tug give it to as soon as he gets to you. More likely, let him just take it away with his momentum. If you use a ball, throw it backwards as he gets to you then release as usual.
Word of caution for those rewarding with balls. If your dog is not those kinds that return to you when he gets his ball, the have another to coax him to remain on track. You might have to teach this 2 ball reward system separately.
Eventually you wean him into the front sit, before the reward and release, as the dog show progression and proficiency in focusing on you for the reward as he gets to you. And voila! There you have it! Hope this helps. Happy training.