Williams %R is a momentum indicator that measures overbought and oversold levels, showing how close the current closing price is to the high-low range over a set period. For example, a reading near -100 suggests a stock like Apple (AAPL) is trading near its lowest price in the lookback period, potentially indicating it's oversold. Williams %R oscillates between 0 and -100, with readings from 0 to -20 typically considered overbought and readings from -80 to -100 considered oversold. Unlike Stochastic Oscillators, Williams %R uses the highest high and lowest low of the lookback period, and the current close, to calculate its value. When the S&P 500 (SPX) drops and Williams %R enters the -80 to -100 range, especially after a prolonged downtrend, it can signal that selling pressure is exhausted and a bounce or reversal might be imminent. Conversely, if Williams %R lingers in the 0 to -20 range for an extended period, it may suggest strong bullish momentum, but also potential for a pullback.