Ok.
it looks crude and it is.
The wooden beam is mounted to a good quality bearing and coupled with a trimpot .
with the pot connected to a cheep oscilliscope .
I use the rig to test individual motor esc combo using b flight motors tab.
Also to see if a motor is week , by spinning each one to the same value in the motors tab.
The torque of motors can be shown by how high the rig bounces when the signal input is bumped by 100.
It bounces because the esc is feeding the motor all the amps it can take for a short period untill the motor speed demanded is reached the bigger the bounce the greater the torque and amp draw.
I usually use it to do a visual and audio test if a quad or motor/esc is playing up.
Can also be used to measure the thrust curve of a motor esc prop combo so if needed the throttle curve in bf can be changed to give the feel or responce wanted.
I hope that answers a few .

Ok, I dig that. How's this: do you ever run the whole quad, and film it from the side, then make benchmarks based on different motor/prop combos? Also: when you are checking one motor at a time, are they the same, or are they all just kinda close to each other? Can you see the effect of things like: worn bearings, bent shafts, loose magnets, demagnetized magnets or bent props? Do you test all your (many) quads against each other to see how much thrust you can possibly make?

Yes all those things and moor.
Measuring things accurately is verry difficult but side by side comparisons aer verry handy.