I have been doing this in reverse for some years now: southward from the chin down, then northward from the bottom of my throat up to the changeover point. It surprised me too to find that my throat beard hairs grow in at least 2 different directions!. . . Turns out that for many men, myself included, the beard grows downwards from under the chin (no surprise there), but then turns 180 degrees and grows upwards for the last inch to two inches going down. Right at the most sensitive part of your neck, the bit that often burns and gets inflamed after a shave. . . .
The solution? Very simple. I start shaving from the bottom up, until I feel the razor start hitting the ATG bit, then stop and shave down from under the chin, again stopping as soon as I feel the direction changing. Takes a bit of practise, but very soon it becomes second nature.
Second pass, I would just go straight from bottom up, to under the chin for the ATG pass. Mostly I don't bother going down again, because I don't want to risk the irritation, so yes the bottom bit is never BBS, but it is irritation free and looks much better than an inflamed turkey neck. . . .