The cdi, or central double increase, is a centered double increase. It takes one stitch and turns it into three stitches.
To work it, knit into the back of the first stitch on the left-hand needle without sliding it off. Knit into the front of the same stitch and slide it off the right-hand needle. At this point, you’ve increased one stitch. Now locate the strand running vertically between the two stitches you’ve just created on the right-hand needle. Insert the left-hand needle into this strand from back to front (or left to right). Finally, knit through the back loop of this picked-up stitch and slide it off the left-hand needle.
The following video demonstrates how to work a cdi: