The helicopter rig is usually used in long distance fishing, or over silt or weed. It is a safe rig to use as the hooklink can detach from the terminal tackle, especially the lead.
Here are some links to useful articles about helicopter rigs on external webpages:
The helicopter rig at Carpland.co.uk
Tie a helicopter rig at Angling Times
VIDEOS:
Watch videos about helicopter rigs-
Jon Bones explains how to construct a safe helicopter rig
Leadcore helicopter rig with Adam Penning
Dan Chart explains a safe helicopter rig from Gardner Tackle