It may have been me as I've posted the Rohloff tool before.

Ironically, all it does is what Dave suggested: it is a new section of chain and a lever. Since the chain is never actually ridden it always stays in like-new condition. Using the tool is simply wrapping the chain around the cogs and applying leverage to see if the chain engages the cogs fully. On new or barely worn cassettes the end of the chain won't rise. On slightly worn cassettes the end of the chain may be a bit loose, but on worn cassettes the chain pops off once the leverage is applied...pretty much like it would while riding.

This tool is in no way the only method for measuring cog wear. I can judge cog wear by eye as many of the more experienced mechanics here can, but what I find it most useful for is proving to a customer that a cogset is worn. If I just told them, they might suspect I'm just trying to sell parts, but when I can show them with their own eyes, they know I'm just trying to make their bike ride better. For that reason alone it's worth every penny, and fortunately it's a tool that should last indefinitely.