My... err... "pecs" get a little tingly when I put my vest on high for too long sometimes.
As for trying this backwards, that is REALLY awesome! I must try that! What a great idea....
And as for doing the vest upside down.... well...
I tried doing a yoga-ish headstand against the wall while doing the vest, I was full flat against the wall on my head, upside down.
Well, lets just say, I would not recommend doing the vest while doing a headstand.