When the glider is approaching the top of a normal launch the cable is at a significant angle to the glider and therefore the glider forward speed will be a lot higher than the cable speed. (a comparison is a water skier doing tight turns, when his forward speed is much more than the boat speed).
To compensate for this, the winch driver steadily backs off on the throttle as the glider approaches the top of the launch.
The winch driver can further cause the cable to back release from the glider under minimal cable tension by closing the throttle completely. The cable disconnection is then barely noticeable other than as a minor click. This minimises stresses on the hook and cable and is also great for early pupils who can be discommoded by an almighty bang if the cable is released by the instructor under full tension.
Full tension cable releases are normally caused by the instructor at about mid point in the launch in order to give the pupil practice in dealing with mid-height cable breaks (i.e. he/she has to decide whether there is room to land straight ahead or turn a turn is necessary after getting the nose down and adequate airspeed.
The sudden release of tension and the "elasticity" of the cable can then cause loose loops of cable to form on the winch drum and/or the cable to fall off the side of the drum, creating a mega birds nest and the winch driver to swear about f****** instructors doing simulated cable breaks!