This trick is as easy as spectacular, especially in beginners eyes.
During the maneuvre, the glider looks like a horseshoe (shape „U”), whilst the canopy is still inflated and the horizontal speed gone. Because in a stable Horseshoe the descent rate is about 6m/s, it was instructed as a possible height-losing manoeuvre (just as big ears or spiral dives), however it was also on the official aerobatics competition manoeuvres list till 2004.
Without releasing the brakes, grab the two middlemost „A” lines as high as possible. Gently and symetrically start to pull down the lines. Now the middle of the leading edge will collapse. As you continue the pulling, the wingtips starts to approach and finally they touch each other in front of you.
It’s almost impossible to do this trick with a DHV 1 or 1-2 glider (low aspect ratio, too stable profile).
Just release the „A” lines and usually the glider opens spontaneously, but if it doesn’t happens, gently pull down both brakes. After it reopened, the glider has to pick up speed, so don’t brake it too much! Anyway, the glider won’t surge, so you don’t have to brake it at all.
However I've never experienced any problems during this manouvre, if there’s any reason, why you can’t open the glider, perform a Full Stall to return to normal flight (of course, only if you have enough height!).
You can get a frontal collapse if you pull down the lines too fast at the beginning.
After the exit, the glider is in deep stall figure for few moments, so let it fly and handle the brakes very carefully, the glider can easily stall!