Downward dog or bow is a great trick to teach your dog which has the added benefit of being a fantastic stretch for your dogs scapula and thoracic spine and muscles, as well as hamstrings.
Teaching downward dog/bow:
There are 2 methods that I use when teaching downward dog or bow
Hand under the belly. Have a handful of treats ready. Hold one arm under the dogs belly to stop them dropping their bottom to the ground. Hold the treats down towards the ground and towards the dog, and mark the behaviour (with a word – I use yes or good) and reward any downward movement, be that head lowering or elbows bending. Once they have their elbows on the ground and the bottom in the air, you can reward the position +++ so the dog gets the idea that when my elbow are on the ground and my butt in the air, I get lots of good treats. Eventually you should be able to remove your hand and the dog maintains the position.
Use the environment. Some dogs do not like having arms under their belly (esp male dogs). In the second part of the video you can see that I use a wall to stop my dog backing up when I lower the treats so that he has to bend over to get them. This was his second session. In the first session every time he lowered his head or bent his elbows I would mark the behaviour (by saying yes or good) and treat him before he had the chance to drop his bottom. I did this very quickly (like a push up). As he began to understand the exercise, I asked more from him and got him to slowly get his elbows on the ground. If he dropped his bottom, I just reset him and started again but made sure I marked and rewarded any downward movement of his front end. Very quickly he gets the idea that I want elbows on the ground and bottom in the air. When he gets the position I reward heavily and release him from the position before he gets tired and drops his bottom.
Do not name the behaviour until the dog is clear about what you are wanting and is successful everytime.