When I am properly stretched and warmed up I lye with my back (with ppt: my back is in complete contact with the floor) on the floor and let my arms relax overhead. I noticed that if I just relax, my arms don't touch the floor. I am able to force my forearms to touch the floor by actively pulling my shoulders back by activating my lower traps. My arms have to be wider then shoulder width to make this possible. Also I am not able yet to let my upper arms touch the floor.
I asked once a flexible kid at my local gymnastic center to do this and noticed that when he went overhead his arms just felt backwards and touched the floor from forearm to upper arm, without any active move: he was completely relaxed (passive)!
My question regarding shoulder mobility (for a handstand) is: when does one have "sufficient" mobility? Is it when one is able to active pull the forearms back to ground or like the example with the kid?