I had no luck in Secret Sessions Forum so I am being a bit naughty and posting in the gaffed deck forum (slightly modified for an open forum)

I have forgotten where I came across a certain trick and would welcome help identifying it. Basically the mage used a pop-eyed popper to force a card. Then using a second deck, a cheek to cheek deck, after face up face down shuffles he was able to unshuffle the deck and reveal the initially forced card.

Thanks for any help from those with better memories than me!