Since nobody has a perfectly straight back, this pattern tapers in with 0.5 cm at the center back of your neck.
If your back slumps forward a bit, fabric tends to pool at the back of your neck. With no sleeves or collar, this causes your neckline to gape. This option tapers the fabric inwards at the back of your neck. It does so by 0.5 cm by default, but you can increase or decrease that value.
Technically this is not a dart, but I wasn't sure how else to call it. Suggestions welcome.