I'll explain the process of using a "classic" rotating mechanical edge finder here; videos are below.
Perhaps the biggest advantage of a "classic" rotating edge finder is the amazing accuracy and precision one can obtain at such a low cost. A high quality "classic" rotating edge finder can often be found for US$20 or less.
Make sure you've got plenty of lighting so as to see the edge finder clearly. One will also need eye protection. A mirror is useful to see those hard to get at locations.
An edge finder is perfectly round with a known diameter. A small section of it is detached from the rest of the body and machined ever so slightly off center - the "wiggely bit". The "wiggely bit" wants to... wiggle as it spins. It sticks out from the main body when it spins.
As one moves the edge finder near an edge the "wiggly bit" will touch the edge before the main body. As one moves closer still, the "wiggly bit" will move closer and closer to the main body (less and less sticking out). Eventually, it seems to "snap in place" and the entire edge finder seems smooth. At that point, one is precisely touching the edge with the center of the edge finder one half a diameter away.
It's AMAZING how clearly one can see the "snap in place". When the edge finder does the "snap in place" it goes "perfectly smooth".
It takes only a few tries - SLOWLY AND CAREFULLY - to see, understand, and internalize this.
One needs to run their spindle for this to work. SLOWLY AND CAREFULLY! Use a very low RPM! Many edge finders are designed with a maximum RPM of 750-1000 RPM. It doesn't take much to see the wiggle clearly. DO NOT EXCEED THE PUBLISHED MAXIMUM RPM!
Precisely because they wiggle, edge finders do not like spinning at high speeds. Too fast and they can literally fly apart, potentially sending shrapnel at the user. Losing an eye is not fun... always use eye protection and stay below the edge finder maximum RPM rating!
Be sure to use decreasing step sizes as you get closer to the edge. The last few jogs should use the smallest increment available.
Using a high quality edge finder - and some practice - it is possible to hit zeroes with great precision. An accuracy/precision of 0.001" (0.0254 mm) or better is easily achievable.
On the machines I've worked on (before the Nomad), one enters a G code directly to the machine - "M03Snnn" (M03 - spindle CW; Snnnn - nnnn RPM; replace nnnn with a number, a slow RPM) - and the spindle will start moving. Then move the gantry/bed around to get near an edge. The rest is documented above. One enters "M05" (M05 - stop spindle) to stop the spindle when they're done.
On my Nomad 883 Pro (and this should work for other Nomaden) this is the procedure for Carbide Motion (CM) version 356 and lower:
From CM screen/interface, click (and connect). Click on the job button. Clock on the "Spindle On" button. Now you can jog and go in and out of the zero screen. When you're done, exit jog and the spindle stops (no need for an M05).
When you're entering an offset (after hitting zero for that direction), enter negative D/2 and hit return.
The C3D edge finder has a diameter of 0.200" (5.080 mm).
BE VERY CAREFUL TO ENTER "M03S1500" EXACTLY. IT IS CRITICAL THAT THE EDGE FINDER SPIN AS SLOW AS THE NOMAD CAN GO (1.5K RPM) - AND NO FASTER!
Yes, the S1500 is below the official Nomad spindle bottom end (i.e. 2000 RPM). I checked with Jorge and it is OK to use this - BUT ONLY FOR THE EDGE FINDER!
Remember that the Nomad does not have reliable repeatability below 0.001" (0.0254 mm).
On the SO3:
Talking with @WillAdams we've determined that all of the SO3 routers (e.g. Porter Cable 450, Makita, Dewalt), whether they have a speed control mechanism or not, all spin much too fast to be able to use the C3D edge finder.
Sorry SO3 users!
The only way to use the C3D edge finder on an SO3 would require a VFD (e.g. SuperPID) to ensure an acceptably low RPM.
And now for the videos:
Here is a nice video showing how edge finders work:
And here is another. It includes some theory behind edge finders and demonstrates just how accurate/precise they can be:
No edge finder discussion would be complete without mentioning high end edge finders like a Haimer. Typically these devices are calibrated strain gauges. Strain gauges can measure INCREDIBLY TINY displacements accurately and precisely. An accuracy of 0.0002" is common with a 0.00005" repeatability. They are calibrated at the factory and unless one does something really nasty to them they NEVER drift.
The probe associated with a Haimer is a precision piece of equipment and is delicate. Use it carefully!
A Haimer and their like are used to measure the edge zero directly - no need to enter an offset. One moves them, much like above, until the tip touches the edge - and continues until the device says it at zero... and it is!
To achieve this kind of ability, these are large devices and require large shanks so they aren't appropriate for a Nomad. They are also in the US$350-850 range. Really, really nice and super effective... a luxury item in some respects.