Yes, chains around the pasterns are intended to alter the horse's natural gait. It is frowned upon by those of us who are against any chemical or mechanical soring methods. Friends of Sound Horses is a good resource to learn more about how bad this can get.
I am betting that they are using the chains in this instance to get the horse to have higher knee action. If you look at videos of Standardbreds racing, you'll see that they are either trotting or pacing, but definitely have a long, straight-kneed type of stride. In other words, they don't lift their feet very far off the ground, and reach way out in front to cover a lot of ground. For some in the gaited world, they want to see higher knee action. The chains (especially if they are heavy) cause the horse to lift his feet higher and further out to keep the chains from rubbing his sensitive pasterns. Some trainers use chemical concoctions on the skin of the pastern to cause the skin to be sore even before the chains are added, which must be excruciatingly painful to the horse when those chains are rubbing and banging against the skin.