Not sure if I posted this before, but I used it this afternoon and it worked so well!

When fitting a new cable on a rear derailleur, first use your hands to push the RD and turn the crank to force the chain onto the 2nd smallest cog -- while the shifter thinks the cable is let out all the way for the smallest cog.

Pull the cable taut with pliers with one hand while tightening the pinch bolt with the other hand.

The pliers will not have been able to pull the cable fully taut, it will have about 1 gear of play.

Spin the cranks and the chain will drop into the small cog, and odds are the shifter will be shifting well across all gears (I didn't even need any barrel adjuster at all today, with a 9sp RD)