Enter your starting investment, how long you held the position, and either your profit or your ending value.
This uses compounding to annualize: it assumes the same growth factor repeats back-to-back over a year.
The exponent (365 / DaysHeld) is the "time-scaled power." It says: "how many holding periods fit into a year."
Raising the growth factor to that power compounds it as if you repeated the same trade back-to-back for 365 days.