This article is from the Hanggliding and Paragliding FAQ, by Joao Geada with numerous contributions by others.
The glider and pilot are mounted on a moveable platform such as
the bed of a pickup or of a trailer in flying position. A payout winch
is also on the platform, and the line from it is attached to the
glider. The platform is driven or towed into the wind, and when the
glider is at flying speed it is released from the platform already
flying. The launch is very much like an assisted windy cliff launch.