The vertical "handshake" position puts your arm in a neutral position for mousing which reduces tension in the wrist and forearm all of the way up through the shoulder. A must have for those who use a mouse for extended periods of time in the office or at home.
Note: After over a month of using this mouse the chronic stiffness and pain in my mousing shoulder is completely gone. See my full review of the Evoluent Vertical Mouse.
From the Manufacturer:
The Evoluent VerticalMouse Wireless is the advanced third generation of the revolutionary mouse from the inventor of the handshake grip. Many users said the VerticalMouse relieved their wrist and arm pain.
Approved ergonomic mouse of University of California at Berkeley
The Evoluent VerticalMouse 2 was evaluated in 2007 by a team at the health services clinic in UC Berkeley, including medical doctors, physical therapists, ergonomists, and administrators. It was found to be comfortable, easily adapted to, and promote a neutral wrist and forearm posture. It is the only vertical type mouse approved for staff use within UC Berkeley and for sale in the campus store.