Grob Glider pilot reported during training maneuver; entanglement of tow line resulted in temporary loss of control with aircraft damage.

Narrative

At a postevent debriefing; the instructor and I and all available witnesses reached the conclusion; after a close inspection of applicable regulations; that an Incident had occurred (not an accident); and this incident was Not Reportable to NTSB or FAA. The flight was a rear cockpit checkout for me; with the club's chief instructor in the front seat. We had planned and briefed with the towpilot that we would do boxing of the wake and slack line recovery on the climb to release altitude. After asking permission of the towpilot upon reaching the appropriate altitude for the maneuver; I commenced: down through the wake; left slide to the low/left position. So far so good; but: coming up from low/left to high/left; I rushed the maneuver and got a little too high above the towplane. I attempted to correct by dipping the nose of the glider down (which causes the glider to start to overtake the towplane); and at the same time; made the error of continuing a right slide to the high/right position behind the towplane. The combination of these inputs causes a slack line to develop; very quickly; a situation that cannot be easily corrected by the usual technique of yawing away from the towline. Also; inertia develops in the right slide; and reducing/reversing bank to slow/end the slide is not readily accomplished either. The instructor in the front seat had wisely popped the airbrakes open to slow the glider; but even this could not remedy the rapidly developing situation. The towline got behind the right wing; when it came taut it whipped us over into an 80 degree left bank; and an eyeblink later; we were 80 degrees nose low. We had not released from the towplane; but; being in a steep nose low attitude with wind noise and airspeed increasing; I surmised that He had released Us. I looked right and saw that the wing was still with us; hence I realized that this situation might have a better ending than initially anticipated. With airbrakes out already; recovery from the steep dive was rather easily accomplished. After a brief discussion; we agreed to get rid of the towline; then returned to the home field for landing. My landing was a little low and long; but uneventful; we could discern no decrement in the flying/handling qualities of the glider whatsoever. After landing; a gash in the right aileron was discovered. Also; the towline was found intact; looped around the right side of the tail of the glider. Avoiding this event in the future includes: not rushing the maneuver; and recognizing early the nonsalvageability of a rapidly developing situation.