Training - Tunnels
Once you have built your dog's comfort with going into a tunnel, both straight and with a bend, these are the skills I think you will want to have.
The first and most used skill is the ability to send your dog to go do the tunnel. Following my standard training process, I first work all approach angles that I will want. From the green handler location around to the orange handler. For this step, I put the handler on the inside of the dog's arc and have the handler push toward the "X" (the 1/2 way point) on the path.
Next I work the distance piece, adding more distance to the send and also more distance between the dog and the handler as shown by the blue path.
Then work rear cross versions of the send - be sure to reward the dog in the appropriate location as the rear cross indicates.
Also work the threadle version of the send - per your handling system. In the picture shown below, the handler pulls the dog in with their off hand and then sends them to the 1/2 way point with the same hand. In your system of handling threads, you may change hands. The set up on the right has a much more dramatic redirect into the tunnel and is worth practicing from lots of approach angles.
Now that you have thoroughly trained how to get into a tunnel, you will want to work on how they come out of one.
When cueing how to perform a tunnel, what I do as the dog is going in, influences how they come out. I will deliberately accelerate as they go into a tunnel if I want them shooting out of it. If I have my near arm (left) directing the dog as they go in, they come out on my left. If I am doing a blind cross at the exit of the tunnel, then my off hand (right) is already presented as they go into the tunnel, showing them they should be working to get to my right side.
Conversely, I will be decelerating and pulling off laterally as the dog goes into the tunnel if I want a turn. And if I want a really tight turn, I will turn into the dog and go on dog-focus as they go into the tunnel, showing collection, exactly as I would on a collected jump. You can think of it as if I am starting my rotation into the front cross as they go into the tunnel (on the left picture). On the right picture I am doing a post turn or blind post turn (depending on which way I turn out of the red handler position)
When using verbals to cue tunnel behavior, my preference is to label the obstacle exactly as I do a jump. For example, I may send them to the “tunnel-left”. If you don’t use verbals this way, as an obstacle name, then at the very latest, call the direction when the dog can see out of the tunnel - not when they emerge.
There are other ways to train tunnels too. In addition to cueing off the handler’s body language as they go in. Some handlers may prefer to train the dog to accelerate through the tunnel and drive ahead to the next thing they see, unless they hear from the handler otherwise. Deciding what behavior you want is important and often overlooked.