The flight instructor of the instructional glider flight reported that during a winch launch with the student pilot, the glider accelerated to 70 kts exceeding the winch launch speed. As the flight instructor attempted to decrease the airspeed to the normal winch launch speed, the weak/safety link broke and the glider separated from the towing cable. The flight instructor attempted to maneuver the glider for a landing back to the runway; however, during the turn to enter the final approach, the glider’s critical angle of attack was exceeded, and the glider entered an aerodynamic stall/spin. The glider impacted trees and terrain coming to rest on a grass field. The glider sustained substantial damage to the fuselage and to both wings. The flight instructor reported that there were no preaccident mechanical failures or malfunctions with the glider that would have precluded normal operation. The flight instructor also reported that he should have maintained sufficient airspeed to prevent a “stall/spin” at a low altitude.