A convenient, blinky red light on the top of your chute. Might just save your ass.
Calibrating it for ground level is remarkably like playing whack-a-mole for some magic reason. Press the button, then press it again every time it blinks. Then Bob is your uncle. Usually deploys a small, fast-deploying canopy that, while it may not save your limbs, might save your life.
Unless you land in water. Under such a circumstance your situational awareness has been so lax that you deserve to survive the jump only to drown, or you were unconscious.
Just, please, don't rely on it.
And when in doubt PLF.