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Here's the video Glen mentions. The flex test is around the 2:10 to 2:15 minute mark. They do it on a metal ring for obvious reasons.

http://m.youtube.com/watch?v=3ACJrAI3SxM

This is not a thumbnail test TNT testing the edge for raggedness, dullness, or sharpness. They're talking about testing the blade for flexibility, i.e. thinness of a full hollow, or half hollow,...