I see what you mean, and I don't understand it either. I would think that when you move the working yarn behind those slipped stitches, just give enough slack so that it doesn't pull. And.. maybe that length of yarn is functional.. it would keep the underarms from stretching too much since the body is knit in a ribbed pattern.
I dunno.. I haven't made this sweater, although I plan to. Hopefully someone will come along that HAS knit it.