The certified flight instructor (CFI) and student pilot were onboard the glider readying it for an automobile tow launch. The CFI reported giving the 'take up slack' command to the wing walker, who was steadying the wings. Before the slack was taken up by the tow vehicle driver, the wing walker prematurely gave the launch signal. The tow driver then accelerated and the glider was suddenly launched into the air. The CFI attempted to control the glider by applying full forward elevator and simultaneously attempting to release the tow line. The glider struck the ground with the line still attached and was again launched into the air as the tow truck continued to accelerate. The tow line then released and the nose of the glider struck the ground. The glider sustained substantial damage to the aft fuselage structure during the accident sequence.