The tow plane pilot reported that, during takeoff for a glider tow, about 20 to 30 ft above ground, the airplane lurched forward and pitched nose down. He added that he pushed the tow line release mechanism forward, but it did not feel as if the tow line released. The airplane impacted the ground in a nose-low attitude, exited the runway, struck a runway light and runway sign, and ground looped. The pilot of the glider being towed reported that, during takeoff, the glider became airborne and abruptly pitched into a steep, nose-high attitude, and he had lost sight of the tow airplane. He released the tow rope, maintained level flight attitude about 150 ft above ground, and landed on the grass adjacent to the runway without further incident. The glider pilot reported in a follow up telephone conversation that the glider used a center of gravity (CG) hook set up for the glider tow. The tow airplane sustained substantial damage to the right wing. The pilot of the tow airplane and glider reported that there were no preaccident mechanical failures or malfunctions with the airplane and glider that would have precluded normal operation. Federal Aviation Administration's Glider Flying Handbook, FAA-H-8083-13A, contains a section titled "CG Hooks" which states: A CG hook, as compared to a nose hook, makes a crosswind takeoff more difficult since the glider can weathervane into the wind more easily. In addition, a CG hook makes the glider more susceptible to kiting on takeoff, especially if the CG is near the aft limit. This can present a serious danger to the towplane during the aerotow. It also contains a section titled "Aerotow Takeoff Procedures" which states: One of the most dangerous occurrences during aerotow is allowing the glider to fly high above and losing sight of the towplane. The tension on the towline caused by the glider pulls the towplane tail up, lowering its nose. If the glider continues to rise, pulling the towplane tail higher, the tow pilot may not be able to raise the nose. Ultimately, the tow pilot may run out of up elevator authority.