SPDR S&P 500 (SPY)

[loanwolf]

You've prolly used it before as Williams is the exact inverse of fast stochasctic.
Plot a fast stochastic of same time period just below and you will see.
It's a mirror image without the moving average (or %D) plotted with inverse negative values.

I believe its due to the variables of the calculation being juggled a bit and being multiplied by -100 vs 100.

All in all you end up with the exact same plotted line.