I'm not sure I understand your confusion - the pattern states:
Divide for Neck Opening: Dec Row (RS): K1, SSK, work to center point marked on chart, join another ball of yarn and work to last 3 sts, k2 tog, k1 – 40(44-47-48-50) sts each side.
Maybe you don't understand SSK? It's slip the next 2 sts individually, then knit them together. Or you can use slip 1, knit 1, pass the slipped st over. That makes a decrease that leans left; the k2tog inc on the other end of the row leans right.