Thanks Tu.114.
Clearly I was thinking in terms of kite flying when I was considering the tension on the cable at release! Obviously the glider pilot is not a helpless participant in the process and can take the tension off the cable with some forward stick before releasing. Obvious after it is pointed out.
Yes a hydraulic clutch makes sense too. Perhaps a simple temperature gauge could save some heartache with melted fuses?
My guess is that the poor winch driver is a glider pilot himself/herself most of the time? Just taking a turn on the rota.