[raldi]

Or the wood is soft and compressible when wet, but returns to its original shape when it dries.

Some woods (including pine) are much more compressible than you might expect: http://www.ewpa.com/Archive/2006/aug/Paper_306.pdf

[dfc]

My favorite example of this is the "nail in wood block trick."

Youtube: https://www.youtube.com/watch?v=fEATei2wewY

Instructions from the patron Saint of Woodworking: http://www.woodwrightschool.com/downloadable-plans/tooth%20a...

[jmpe]

you can bend/compress wood with steam. But look at the width of that bottle neck.

The others I looked at (playing cards, rubicks cube, ...) can be disassembled in smaller pieces that go through. That's his technique. So how do you disassemble a plank? I think it's a hollow piece of thin plywood (1 piece, no seams) that was soaked, folded up and then filled with something (resin).