The Williams Acceleration/Deceleration indicator is a simple MACD Histogram 5, 34, 5 of the mid point of the price bar.
It is traditionally plotted with up bars in green and down bars in red.
|Only Plot Moving Down||
The Divide by 0 error in the Only Plots Moving Down formula is intentional. It makes it so the red bars do not plot when the AO is not moving down.