SP2P stands for slip one stitch as if to purl, p2tog, then pass slipped stitches over.
This decrease is a left-leaning, double decrease, where you slip one stitch as if to purl, p2tog, and then pass the slipped stitch over the p2tog. This decrease turns three stitches into one (decreasing two). One stitch is decreased in the p2tog and one by passing the slipped stitch over. It is the purl version of the SK2P, slip one stitch, k2tog, pass slipped stitch over.
Working an SP2P
Slip 1 stitch as to purl, p2tog, then going into slipped stitch from left to right, use the left needle tip to pass the slipped stitch over the p2tog and off the right needle.
