http://www.smith-wesson.com/misc/ilock.html
Smith & Wesson customers will soon begin to see revolvers with a new feature,
an internal lock designed to limit unauthorized use of the handgun. The new
system features a locking mechanism that is built into the revolver. A small
entry port for key insertion is located above the thumb piece on the
left-hand side of the revolver. The key has a special configuration designed
to maintain the firearm's security by significantly reducing the possibility
of tampering.
Turning the key approximately 90 degrees to the locked position blocks both
the hammer and trigger, rendering the handgun inoperable. Exposed hammer
guns have a lock status indicator that protrudes above the frame in the area
of the hammer to indicate when the gun is locked.
When the gun is locked, neither the hammer or trigger can be moved, but the
cylinder of the revolver can be opened and closed to insure the revolver is
unloaded. When not locked, no part of the lock mechanism is in contact with
any of the firearm's working parts and therefore the lock does not change the
feel or function of the Smith & Wesson revolver action.