Help me remember this party trick about splitting the brain's attention

I can’t quite remember it, but it was along the lines of the party trick where you stand in the doorway and with your arms at your sides, you push outward against the door frame. and after a minute, you step out, let your arms relax and your arms float up.

The trick I’m trying to remember is something about putting both arms out and then having someone push down against your right hand. Once you reach stasis (pushing at equal strengths) the person(or someone else) can reach out and with minimal effort push UP on your left hand, and your right hand will immediately lose it’s battle.
I can’t try this out from work, but I think I have some part of the trick wrong. Does this ring a bell with anyone else?

I’m not sure if I am right about one hand being pushed down while the other is pushed up. The only thing I remember for certain is that the original hand loses strength when your brain has to concentrate on another body part.

Well, I remember this part of it anyway. Stand in doorway, arms hanging limp at your sides, hands open, palms facing toward you not outward away from you.

Now, press the back side of one hand outward against the door jamb. Use the muscles of your [del]upper arm[/del] shoulder (I guess) to do this. Don’t just lean your body against the door. Do this for a minute or two.

Then let that arms hang limply again. The upper arm muscle that you were using to press against the wall (the deltoid?) will tighten up, causing that arm to float upward.

The rest of your trick, I haven’t heard of.

Yes, I remember that one, but the other one is still a mystery. Anyone?

I can only find a trick that’s a little similar to what you describe.

That’s quoted from this page -

That’s not quite it, but neat nonetheless! Thanks!