Asana​

Koundinyasana II - One Leg Koundinya's Pose
Arm Balances - Splits

There are considerably more arm balances with bent legs than straight legs. The more the weight of the legs extends away from the body the more strength the pose requires. The leg which moves to the arm also needs lots of mobility to achieve the final position. If the mobility in the leg is not yet there you can work on poses like Mandukasana, Wide Leg Forward Bend, Standing Split, Trikonasana, and Snail Pose. For the upper body strength a solid, well aligned Chaturanga is required. Even once the legs lift to Koundinyasana, the shoulders should maintain close to the height of the elbows. 

Progression

Start from downdog split and work on bringing the bent knee towards the arm and partially bending the arms towards chaturanga. 

Final Alignment

Both legs straight
One leg resting on the back of the upper arm
Shoulders in Chaturanga, as high as your elbows
The whole body can be kept parallel to the floor, or lifting higher to work the back muscles

Assisting

Hands on adjustment is not so practical in this pose. Sometimes you encourage lifting the shoulders with your hands. It is better to check alignment and offer variations for those who are not yet in the pose in a beneficial way.