Actually, i didnt train him to do that. He taught me. He just walked up to me with his toy in his jaw. I assumed he wanted to give it to me so i grabbed the toy. Only he didnt let go of his end. Instead, he began to shake his head vigorously while keeping his teeth clamped down on the toy. I couldnt hold on for long for fear of ruining the toy, so I let go. Second i did, he approached me again with the toy clamped in his jaw, expecting me to grab on again.
My black lab does that too but she doesn’t initiate. I consider it an instinctual throwback to dogs fighting over the remains of their prey.
Don’t take it as harmless though. It may indicate the dog considers you a competitor for resources. I don’t believe that is healthy for a dog’s relationship with people.
After I do a tug of war with my dog I always make sure to switch to a more cooperative game which makes her dependent upon me to initiate the action (throw the ball, etc.). It helps to remove the competitiveness between us.
Oh, and I ALWAYS try and win the tug of war. Whenever she gets dominant at something, like swimming in the pool, she becomes a total pig and can take it too far with me or the kids. Thankfully, she now knows the full meaning of the word “dumbass” so I can set her straight with a glance and a word.