So the basic mechanism that this is using is the "reflected endpoints" of the construction line.

If you drag out a construction line with the mouse down starting point and the mouse up release point like this:




The distance between those points is used to create additional End snaps, extended by the same distance on the other side, like this:






When applied perpendicular to a central axis that gives a mirrored point around the axis.

You can also use that distance in a couple of other ways too, like "capture" a distance between 2 points and reapply that distance elsewhere by holding down on the little menu launcher tag and picking relocate or reorient cline, and also you can put in a division number on that little menu to enable "div snap" to do things like snaps at 1/5 or whatever you want increments along that distance.

- Michael