TRIX: the one-bar rate of change of a triple-smoothed log price.

Working in logs is what makes the output a pure percentage rate (scaled by 10000, so a reading of 100 is one percent per bar) independent of price level, and three EMAs in series strip nearly everything that is not the underlying trend, which is why it crosses zero so cleanly and so late.

That lateness is the cost: each EMA starts length - 1 bars after its input, and change adds one more, so the first print is at 3 * length - 2 (bar 52 on the default 18).