Man, that drifting is the worst! I've had that happen on big jobs and it kills your back trying to fight it. Using a clamp on the runner head like that is a solid trick, locks everything in place so you can focus on the stretch. Saves so much time and hassle in the long run.
Hot take: Genius move.
That drifting is a total nightmare on long walls... makes the whole job ten times harder. I've seen guys fight it for hours and still get a wavy line. Clamping the runner head is such a simple fix, it's one of those things you wish you figured out years ago. Saves your back and your sanity.