The concept is measuring inertia with the ema (it reacts faster to recent price changes than sma), and momentum with the macd histogram representing bulls and bear "fight". When MACD Histogram and EMA are both rising, the histogram bar is coloured green. When they are falling, the histogram bar is coloured red. When they diverge, the histogram bar is blue. When green, go long or stand aside. When red, go short or stand aside. When blue, go long or short, or better stay away.
hope someone will find it useful