Two indicators – same principle. Combine the current price with an exponential moving average (EMA), which is a trend-following, lagging, indicator.

Bull power = Current High – EMA of closing prices
Bear Power = Current Low – EMA of closing prices

Both of these are then plotted as histograms. The exponential moving average is usually 13-period as a good compromise.
See a (5-min) chart of Bears power on GBP/USD.

The theory runs that these indicators, though simple, are a method of spotting overbought/oversold conditions, and therefore going against the trend – a professional decision when all the ‘amateurs’ are going one way, knowing it won’t last long, to go the other way – slightly higher risk…

For example, a Buy signal: Trend is up as shown by EMA and Bear Power is negative but rising. (Bear Power moving higher from a bullish divergence strengthens a signal).
Sell signal – the reverse.

Bulls and Bears Powers can be combined into an oscillator and form the basis of the Elder-ray indicator, more popular in securities etc. than in the forex market.