The Range Expansion Index (REI) indicator is an oscillator which measures the market price changing rate and shows you overbought or oversold areas, in case a market price shows weakness or strength. The indicator was developed by Thomas DeMark and mentioned in his book "The New Science of Technical Analysis".
The indicator values change from -100 up to +100. REI is an enhanced oscillator: it stays neutral while a flat period and shows you trends only when considerable top or bottom have been reached.
Thomas DeMark redommends using the default period equal to 8. It is time to open a sell order if the price crosses level 60 upwards and then goes down and crosses this level downwards. It is time to open a buy order if the price falls below -60 and then goes up past this level.
You can download the indicator here: https://www.mql5.com/en/code/529