This is the Two Pole Butterworth Filter described in Chapter 13.  Ehlers introduces Butterworth filters as better filters than exponential moving averages.

PriceIn = defaults to Close but change to the Price indicator to match the book

Period = defaults to 15 as described in the book

For more information on this indicator, refer to Chapter 13 of "Cybernetic Analysis for Stocks and Futures" by John Ehlers, Publisher: John Wiley & Sons; (March 19, 2004), ISBN: 0471463078