effect is very handy when you don't have whimsical panties
/tights: just wear 2 tights (same brand, black ones 20 or 30 denier for the best effect) on top of each other and the pattern
changes while moving your legs
I don't think Moire invent
ed it with that purpose.
I tried to explain this Moire pattern to a collegue, but I didn't want him to fixate his looks on my legs constantly, so I'd showed him http://www.sandlotscience.com/Moire/Moire_frm.htm , wich has some interactive examples.