This is the main reason that Kanku-sho is a popular tournament kata, giving the contestant the opportunity to show off an impressive high spinning jump, with an ushiro-geri in mid-air, and often to applause and whoops from the spectators.
In actual practice, according to Nakayama, the jump should not be a high one. It is really a fairly level spin (though the left foot does leave the floor) into a sudden drop to the ground, thrusting the left leg out into ushiro-geri before landing. A jumping spin in Shotokan represents a throw.
It is a very hard move to master. The points I am trying to incorporate include hitting the mikazuki-geri against the open hand directly behind me (ie. not moving the open hand further round once I start turning), getting the ushiro thrust out, and landing with my hands and feet in the right position. Not to mention then springing up into the low gedan shuto-uke! My worst fault is not controlling the aftermath of the mikazuki, which often carries my ushiro leg round too far on landing.
It is important to note that in Kanku-sho, in ryote-fuse, the left extended leg is positioned on the ball of the foot, different from Kanku-dai where the foot is sideways on and flat on the ground.
The old version of the kata (Kushanku-sho) didn't have this move as a jump, it is a spin with the foot planted resolutely on terra firma. Some people say that the jump was included by westerners purely to bring more glamour to competition karate, but as we know from Mitsusuke Harada, he described seeing Kanku-sho in the 1940s complete with the Shotokan jump. I think this points to it being a development by Yoshitaka Funakoshi, and here is a quite impressive photo of him doing the tobi ushiro-geri from Kanku-sho …
The application that I'm demonstrating for this technique is to step away from a close-quarter grab or punch, while parrying with the haishu-uke. With the open hand I grab the assailant's arm and twist it slightly so my mikazuki-geri strikes hard against the locked elbow joint. Keeping hold of the arm I then turn quickly into ushiro-geri, thrusting my heel into my attacker's mid-section at close-range.