The original script was posted on ProRealCode by user Nicolas.
It's based on the Repulse indicator which is an indicator that shows the bullish or bearish pressure on the price at any one time. It is particularly significant when it crosses above or below the 0 line because this indicates the start of a new bullish or bearish trend. The Smoothed Repulse indicator is obviously a smoothed out version of this indicator and it does seem to illustrate the trend extremely well.