In this variation, the side crow or parsva kakasana in which one or both of the legs extend out, the legs, too, move with little gripping, tension or muscular might. In fact, it's a neat sensation to feel just how light the leg(s) are in this pose. It's almost like they're floating, and confirms yet again that this pose is one of balancing, not strength. Looks can be -- and are often -- deceptive.
Preparation for the side crow is different from the basic version, kakasana, in that you want to develop an active twist in the middle.
You can spend days, weeks, months just perfecting a twist that allows you to rest both elbows on your thigh. Once you're comfortable with that, then drop the palms down flat on the ground, fingers pointing forwards, head and neck down and stretching out. You have to get used to really bending the elbows and flexing the wrists, hunkering down into the side.
You have to get used to the sensations from all this hunkering down before you even think about supporting the thighs on the arms. Then, rock to the side and back, shifting weight from the palms back to the feet, from the feet into the palms. Again, you may need to just rock and rock for days (weeks, months) before you discover the key to the movement, the key to your flight in space even if it is only a few inches above ground.