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»).