Barbara Walker was the first knitter to document this stunningly simple method of knitting in a sleeve from the top down and I always use it.
It's definitely worth the time to learn. No lumps, bumps, or seaming and no complicated math. The best part is that it fits every single time without fail.
If you boiled down the technique you would see that it is very similar to turning a heel on a sock or working creasant shaped shawl like Annis.
I'm not going to write a tutorial but I will give you a brief pictorial.
Pick up the stitches around the armsyce.
Work back and forth from the center down to the underarm.
Take a closer look at the faux seam.
Is'nt that brilliant?