This technique (called Nognitsy in Russian) is a staple of sport and combat sambo. this has to be my all time favorite takedown mainly because it requires very little set up or off balancing to work. I also like that it can lead directly into leg attacks. This technique is seen in many, many martial arts because of its simplicity, so it is no wonder it found its way into the very diverse yet succinct system of combat sambo.
take a grip on your opponents collar. it is important to take the grip with the arm corresponding to their lead leg. so if their right leg is forward you need to grab the collar with your left hand.
jump into the air and slam your lead thigh into his stomach. you should rotate your body so you are facing the same direction he is.
throw your back foot off the ground behind him, and put it strait out behind his knees. rotate your body toward him with your legs strait. this rotation will cause the scissor action forcing his hips and his knees to bend.
turn toward your opponent to finish the takedown and force him onto his back. for safety it is very important that you rotate your body in mid air and try to land facing your oponent. notice that my right hip does not hit the ground in this technique. if i was to set up the scissor and fall strait down without rotating i could severly damage my opponents ankles. you need to force him to fall backward by rotating. (note* this technique is not legal in judo because of this danger.)