They can either clamp to the desk and support the forearm(s) in a static position or others support the forearm so that the whole shoulder is encouraged to move while mousing.  These forearm supports can be an alternative where there are no arms on a chair and the user is experiencing shoulder and neck tension.

For those who use a mouse for much of the day, an forearm support is just the thing to support your arm and your shoulder while you are working.  For instance, a proper forearm support alleviates stress on your spine and allows you to keep your wrist aligned. The benefit of using an forearm support is that you are able to be just as efficient, while keeping everything aligned correctly and avoiding the strains that cause things like Carpal Tunnel Syndrome or Repetitive Stress Injury.

The more ergonomic forearm supports will encourage full movement through the shoulder and arms while supporting the arm and should while working.