The body is knit in the round from the bottom up, with slipped stitches forming faux seams. You then cast on at the cuff of one sleeve, and when the sleeve is finished being knit in the round, you separate it out and start knitting in rows. This then becomes the yoke, which is attached to the body as you go along. It's great, except that when you do this, you end up with an even larger-than-average hole at the armhole.
Yuck! Luckily, I remembered TechKnitter's simple solution for this: crossing stitches. The X that's formed helps to take the strain better and maintain stability.
This simple maneuver makes a huge difference! I also used this trick where the yoke stitches were separated to create the neckhole.