The pilot and flight instructor planned on conducting a flight review in a glider, which was expected to take three flights. The first flight was the accident flight. After departure, the pilot performed several maneuvers behind the tow plane, to include 90-degree steep turns, before releasing from the tow plane. The pilot performed some additional maneuvers before descending to 1,500 ft agl in preparation to land. The flight instructor reported that he gave no specific instructions to the pilot for the landing. As the glider approached the runway, the pilot preformed a forward slip with full rudder and full spoilers deployed. The pilot stopped the slip in order to clear trees at the end of the runway. After clearing the trees, the pilot resumed the slip with the spoilers still deployed. As the glider neared the ground, the pilot discontinued the slip. The flight instructor said that as the pilot came out of the slip, the glider's descent rate increased. The pilot made no attempt to flare the glider, which resulted in a hard landing. The glider sustained only minor damage; however, the pilot was seriously injured. The flight instructor stated that an upslope in the runway may have given the pilot the visual illusion that that they were higher above the ground than they actually were. He also said that he was overly confident in the pilot's ability and let his guard down.