The pilot was approaching to land at the conclusion of his second flight on the type. The approach was normal, however the glider floated the length of the strip at a height of about 25 feet. At the end of the strip a turn was commenced but after completing about 90 degrees an incipient spin developed and the glider struck the ground. The pilot subsequently advised that he had activated the cruise flap lever instead of the air brake. The two levers are located side by side in the cockpit. It is likely that the pilot was suffering a degree of heat stress. He had been in the cockpit, exposed to the sun for two and a half hours and he had not eaten since breakfast. He reported that during the final stages of flight his reactions were affected by fatigue and the heat.