the challenge of raw dan tien power: dan tien rotations; and standing

‘small soft rubber ball moving inside dan tien’

Put hands on belly.
Let him/her do the dan tien movement.
?
If recognized above is a sign of achievement.
If not keep going.

This standard is endorsed by Jeff Smoley of http://www.americanchentaichi.com and Kosta Daneos book 1.

Trunk’s supta asana is opening kwa first and legs second.
This is rare. General cross siting asanas are lock thigh/upper leg far to easily and need adaptation (lots padding) to enable reaching the psoas strech before locking leg tissue.
However I prefer the front stretch (a la kumar spinal stretch). Lower back is already usually compressed in back stretch.
I have been studying standing in the light of cultivating dan tien. And developed following procedure:

1.Knee pads above the k1.
If knees bent further it causes knee strain unless kwa is already very strong.
However feet can be at varying distance.

2.Tilt the kwa down and open the ming men: lift the kwa.
This action is usually accompanied by contraction of front point:
ming men opens, front point closes.
It is better that the front of dan tien closes then that the heart closes.

3.Release the contraction of front point (that happened in 2.)

Then all else is for fun.