Having a tool to measure the speed and change of the price movements is invaluable to those working in stocks & technical analysis, which is why John Welles Wilder Jr. came up with the relative strength index (RSI) all the way back in a 1978 book published by him.But what is RSI, and how is it used? To help you figure that out, I’m gonna go into detail on this system and help you better understand it.