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

Acceleration/Deceleration Oscillator (AC): Buy below the zero line / sell above the zero line

If the histogram AC is below the zero line, a buy signal is generated when there are three consecutive green histogram bars. Place a Buy Stop one tick above the high of the price bar that corresponds to the third highest high (“D”). If the histogram is above the zero line, a sell signal is generated when there are three consecutive red histogram bars (pic.1). Place a Sell Stop one tick below the low of the price bar that corresponds to the third lowest low on the histogram (“D”).
Picture 1. Acceleration/Deceleration Oscillator (AC): sell signal
If histogram “B” or “C” bar crosses the zero line, then histogram “C” bar becomes a signal bar, and there is no need to wait for the “D” bar to occur. Bear in mind that no matter what signal is created you must ignore it until the first fractal buy or sell signal is triggered outside the Alligator's mouth. You should ignore signals that are not in the same direction as the first signal from fractal. If Acceleration / Deceleration Oscillator generates a signal but the histogram changes its colour before the pending order is executed, then the signal has to be ignored and the order has to be deleted.

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

Acceleration/Deceleration Oscillator (AC): Buy above the zero line / sell below the zero line

A buy above the zero line signal is generated when there are two consecutive green histogram bars (a green histogram bar is a bar whose high is higher than the preceding bar's high). The histogram is above the zero line. (pic.1). Place a Buy Stop one tick above the high of the price bar that corresponds to the second highest high («C»).
Picture 1. Acceleration / Deceleration Oscillator (AC): buy signal
A sell below the zero line signal is generated when there are two consecutive red histogram bars (a red histogram bar is the bar whose low is lower than the preceding bar's low). The histogram is below the zero line. Place a Sell Stop one tick below the low of the price bar that corresponds to the second lowest low («C»).

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.