http://www.alpari-forex.com/en/chaos/ac.html

Acceleration / Deceleration Oscillator (AC) overview

Acceleration / Deceleration Oscillator (AC) measures the acceleration and deceleration of the current momentum (the third market dimension). Bill Williams says that before the price behavior changes, the momentum changes and, even before the momentum, we see the change in the acceleration. Acceleration / Deceleration Oscillator (AC) histogram in MetaTrader 4 is the difference between 5/34 momentum histogram (Awesome Oscillator) and a 5-bar simple moving average on the Awesome Oscillator: MEDIAN PRICE = (HIGH + LOW) / 2 AO = SMA (MEDIAN PRICE, 5) — SMA (MEDIAN PRICE, 34) AC = AO — SMA (AO, 5) Where: HIGH — the highesy bar price; LOW — the lowest bar price; SMA — simple moving average; AO — Awesome Oscillator. In order to add the Acceleration / Deceleration Oscillator use the “Insert -> Indicators -> Bill Williams – Acceleration Oscillator” menu sequence.
Picture 1. Acceleration/Deceleration Indicator
Unlike the Awesome Oscillator, if the Acceleration / Deceleration Oscillator (AC) crosses the zero line, then it is not a signal. However, we still never buy if a histogram bar is red and never sell if it is green.