i trust you are preparing a 32-slide presentation for SPY outlining all the clear indications that SPY better get its sagging butt back up above the 205 level by weds or tickers are gonna roll...
probably best to administer the presentation during premarket on monday so it's fresh in SPY's mind if SPY gets pummeled by some piling-on dumping at monday open.
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.