Hopefully this'll take some of the guess work out of what I'm describing.
The following video shows the exercise being done, towards the end, the person on the left holds him down slightly, without exerting visible pressure.
I don't think the connection is at the shoulder, in fact I think the spine plays a more prominent role, and needs to be coordinated with the tanden. Its not "just" the tanden.
Its simply an exercise in understand what it means to "stand" and transmit your weight without committing it.