Its a device that measures vibration. Hahahaha
They use a accelerometer to measure vibration and its calibrated for up on the sensor and tells the rpm of the vibration. And has a strobe light line to show the location.
Then add weight to smooth it out. The key is finding the worst location at a given rpm. In my case the worst vibration was at the pinion then it was a toss up where tonstart. I started at the driveshaft only slightly changed the measurement, then added weight to the yoke and made a jump. Checked run out and no change
https://www.ebay.com/p/Kent-Moore-J-...er-2/656720445