" In technical analysis, this is a momentum indicator measuring overbought and oversold levels, similar to a stochastic oscillator. It was developed by Larry Williams and compares a stock's close to the high-low range over a certain period of time, usually 14 days. "
"The failure to move back into overbought or oversold territory signals a change in momentum that can foreshadow a significant price move. The ability to consistently move above -20 is a show of strength. After all, it takes buying pressure to push %R into overbought territory. Once a security shows strength by pushing into overbought territory more than once, a subsequent failure to exceed this level shows weakening momentum that can foreshadow a decline."
( from http://stockcharts.com/school/doku.php?id=chart_school:technical_indicators:williams_r)