In de video you can see the 4-years old Frederiksborger gelding Prince Elmelund:
Work at hand or groundwork makes the horse obey and cooperate with the rider. With groundwork exercises, the horse learns to respond to the aids and it develops proper muscles. This prepares the horse for longeing and riding. Groundwork is done using a cavesson and a lead rope or longeing line.
The line is attached to the middle ring of the cavesson. In this way, the young horse is easy to control, without harming/disturbing the mouth.