The sag indicator is simple tool to measure the amount of travel you are using on your coil shock, and to get the ideal sag measurement.
An external shaft is held in place at the top and bottom of your shock, with an "o" ring to show how much travel you are using.
The Travel and Sag Indicator can be used on the bike while testing and setting up your bike, zip tie it on, do some runs and see how much preload you need or if your spring rate is right.
For setting sag, simply measure where the "O ring" has reached with a ruler and compare to a sag chart. This saves time and is much easier to get an accurate reading.

https://reverse-components.com/en/products/travel-and-sag-indicator