Central Double Purl Decrease (cddp)
The cddp, or central double purl decrease, takes three stitches and reduces them down to one. Instead of leaning to the left or the right, a cddp remains centered. On the opposite side to the one it's worked on, the cddp looks like a cdd.
There are several ways to work a cddp. This is the method I prefer: Insert your right-hand needle into the first stitch on the left-hand needle as if to knit and slip it onto the right-hand needle. Do the same with the next stitch on the left-hand needle. At this point you've moved 2 stitches to the right-hand needle by slipping them knitwise. Next, slide the left-hand needle into these 2 stitches from right to left and slip them back onto the left-hand needle together. Finally, purl these 2 stitches together along with the next stitch on the left-hand needle. (You will be purling 3 stitches together.
The following video demonstrates how to work a cddp. It begins with a cdd, with the cddp demonstration beginning at 0:26.