What causes my driveshaft to "jump" in this video?

Guess you can call me a back yard mechanic. I have cut several wrenches to make a specialty tool. Still have it and still use it, a spray paint can lid, drilled a hole in center, insert a rubber (clear) hose and cut the end of a large nail and installed in the other end. Use the nail as a probe and the lid to my ear to find bad bearings and such. Just a thought, with little or no costs. Got the idea from the telephone as a little tot, two cans and a string. Sure am showing my age. LOL