The glider winch-hook has a back-release mechanism, which should operate as the the glider overflies the winch. If that doesn't work,then the winch driver uses a guillotine to cut his end of the cable.
The subsequent landing with >1000' of cable attached should be "interesting"
From a design point of view, the winch launch is a very important case. At the top of the launch, the wing has to generate lift equal to the weight of the glider PLUS the tension on the cable.
Overlap with Above The Clouds, who points out the weak link which I forgot about!