This article is from the Hanggliding and Paragliding FAQ, by Joao Geada with numerous contributions by others.
Have you ever flown a kite where you run along paying out string
from a ball as you ran while the kite climbs? Similarly, a payout
winch is a winch which is mounted in the back of a truck or on a
trailer, and pays out line as the hangglider gets higher. The line
tension is maintained by the use of a disk brake from a motorcycle or
car which is mounted on the side of the winch drum. The amount of
drag the disk brake exerts is controlled by the winch operator but if
set generally remains constant for all payout speeds.