As was mentioned in tutorial 98, a narrowing of the Bollinger bands is an indication of lower volatility. According to John Bollinger (who created the Bollinger band concept), periods of low volatility are often followed by periods of high volatility. This could mean that a narrowing of the Bollinger bands (or a period of low volatility) --a 'squeeze'--may foreshadow a period of higher volatility, or a breakout in price.
Another way of looking at expansion of volatility is when the Bollinger bands cross outside a Keltner channel.
Program 59 searched for both constriction in the height of the Bollinger band and where the Bollinger bands move outside the Keltner channel. It actually includes two programs:
_Program59 (an indicator)
_Program59 PB ( a paintbar study)