Because showing incorrect movements and possibly misrepresenting freerunning is worse than not showing moves at all.
Word.
And to the actual question...have one of your friends bend over and practice rolling sideways over his back. Then actually put some force into it and jump, you'll probably still be touching his back. Once you have that down, jump (you should really block, if you don't know what "blocking" is look it up on youtube) and torque your upper body. When you complete the rotation, you'll be rotating around your back mind you, and land on your feet you've just completed a double leg. At first your legs will probably be bent but eventually they should be piked out.
The reason I suggest learning a double leg first is because its A.) Really easy B.) Really safe...you're only rolling over your friends back in the worst case.
So yeah, go learn a double leg today.