Jose wrote:
How is this cable used, and why would it extend up three thousand feet?

have you played with kites when a kid? a bit the same way, but
using a very powerful engine instead of just running holding
the wire (though that technique, running holding the wire,
can also be used, albeit towing it with a car, done it as
well :-) The thing about the warning the previous poster
mentioned: the glider can be sitting on the ground still, and
a minute or two later, be flying full speed at 3000' (rates
of climb to kill for :-); which could be a surprise to the
unsuspecting power pilot overflying the field already, but
the cable might be even more of a surprise as well...
(seriously, if you have a glider port with a winch in your
vicinity, and you never experienced a winch launch, ask for a
ride!)

--Sylvain