Going back to the original question of the risk of the glider being unable to release the winch cable.
Clearly that would be a pretty serious situation even given the presence of cable guillotines on the winch etc.
The automatically releasing hook ("back releasing" hook) was invented pre WW II to counter this risk and as has been said, its action should be checked daily before the first flight. It's a very reliable system even if (say) the cable release knob comes off in the pilot's hand.
The remaining risk of a winch cable "hang up" these days is if at the start of a launch, after a slightly enthusiastic taking up of the slack in the cable, the glider rolls forward, runs over the cable and picks it up around the mainwheel.
This risk is countered by the last few metres of the cable ("the strop") near the glider being encased in a relatively stiff length of plastic hose. Also the launch should be aborted by the signaller if the glider is seen to overrun the cable at all prior to the "all out" signal being given to the winch driver
All these security systems have evolved over the last 90-odd years since the gliding movement took hold. The risk of a "hang up" is one of the first things an onlooker thinks about when he first sees a winch launch, but in reality the risk is now tiny (never say never)