At an altitude of about 4,500 ft the glider was observed to pull up into a stall and enter a spin which continued for three turns. The glider came out of the spin into a left spiral and appeared to be traveling at a very high speed. The nose then 'tucked under' until the glider was inverted. What appeared to be a two to three turn inverted spin followed. The glider recovered from the spin and flew inverted for a short distance. It then appeared to roll upright briefly, but returned to the inverted position before impacting the ground. The flight manual which accompanied this aircraft advises 'The 1-23H or 1-23H-15 is aerobatic, but extreme cautions should be taken not to exceed placard speed. Pilots with no aerobatic experience should not attempt inverted or difficult maneuvers without an adequate amount of dual in such.' Under the topic of spins the manual also stated 'Recovery is normal. Care should be taken not to dump stick too much as ship may go beyond vertical.'