When the sling was added to the trebuchet it gave the tossed
projectile more velocity. This meant the trebuchet could send
a lighter projectile farther, providing more distance between
those who fired the machines and deadly enemy archers at the
castle.
Here's how it works. The sling lies on a track at the base of
the trebuchet. As the arm of the trebuchet begins to move, the
sling drags the ball before lifting it into the air. This
gives the swinging arm time to pick up speed.
The sling is an extension of the beam's reach. At the time it
is launched, the sling is actually moving faster than the
beam, creating a whipping motion. A faster flung stone will
deliver a stronger punch to the wall. With the extra velocity
(which can be converted into a longer throw), the trebuchet
can also be moved back to a safer distance from the castle
wall.
There's an important catch, however. If a sling is used to
extend the range of a trebuchet, a lighter ball must be used
to reach the wall. The problem with a lighter ball is that it
might not have the oomph necessary to destroy the wall.