Theory:
It can be found under different names on the net (MetaTrader 4 version mostly though). It is based on highest high and lowest low of two different periods. Based on those values a trend is derived.
Usage:
You can use close changes as signals.

Channel design indicator
A channel is a trading range between support and resistance levels that a stock price oscillates in for a period of time. This indicator draws the resistance and support lines based on the peaks and valleys that are found within a defined period of time.
Ehlers Fisher transform (original)
Ehlers Fisher transform (as originally described in the book)