The green line is the Average True Range (10 days) and shows us when volatility is high or low. A spike in volatility, larger daily ranges will be accompanied by the green line moving higher and when the ranges tighten up and are smaller in nature it moves lower.
A nice bullish upward trend is usually common with lower average true range as you get a consistent climb without too many wild moves and a bearish move is usually consistent with a larger move in the average true range as volatility increases and move wild moves are prevalent.
Warning signs after a strong uptrend is an increase in volatilty as a topping process is the first step before a decline. This spike in volatility can be a topping process similar to the beginning of decline like in April 2010 or possibly a consolidation phase similar to November 2010. Either way, a rapid move upwards of the Average True Range is the first warning bell your likely to receive.
6 out of the last 9 trading days have seen almost 100 point daily swings with 3 of them over 170 points, so this could be a warning bell right here. If we see this Average True Range continue to accelerate like it did back in April last year we would view this as very dangerous.