Central Double Decrease (cdd)
The cdd, or central double decrease, takes three stitches and reduces them down to one. Instead of leaning to the left (like an sssk) or the right (like a k3tog), a cdd remains centered.
To work it, insert your right-hand needle into the first two stitches on the left-hand needle together as if to knit and slip onto the right-hand needle. Next, knit the next stitch on the left-hand needle as normal. Finally, lift the two slipped stitches (now on the right-hand needle) up and over the stitch you just knit, so that they drop off the needle.
The following video demonstrates how to work a cdd. The explanation and setup starts at 0:31, with the actual demonstration of the stitch at 1:10.