The idea is to sit lightly on the sit, and yes, it is not easy until your legs develop the strength. But, it sure makes you a better rider. You sort of use the seat as a reference point without dropping your entire weight on it.

Of course, learning how to levatate would probably help too...

I am not sure, but I would assume that having clipless pedals is a big help. I find myself moving from sitting, to standing, to sitting, to crouching half out of the seat all the time. Now granted, my roads are not asphalt, but rock and gravel. Coming down hill, I always are on the pegs, not on the seat. Going up some of my hills, if I was on the sit, I would flip over.

To move around like this, you will need strong stomache and back muscles, as well as arms and legs - which is a bonus for fitness.