You're doing two different types of decreases here, the first is the k2tog, the other is the sl1,k1,psso, which is essentially the equivalent of ssk. Sometimes it's abbreviated "skp" or skpo" which (to me) makes it a little more obvious that it's all part of one operation. The important part is that both of them decrease one stitch, they just "lean" in opposite directions (k2tog like / and skp like \... I think). So (from what I can see) it's the combination of the two that creates the "zig-zag". And the YO's create the holes that make it lacy.
Also, since this is a decrease, you'd skip the "sl1" stitch knitwise. I think it also matters that the next stitch is a knit, but it might not. normally if the pattern doesn't specify otherwise, you'll slip purlwise UNLESS the stitch is part of a decrease. Which this is.