With your right needle, come up through the head of the purl bump just below the one on your left needle, from bottom to top.
Place this loop on the left needle without twisting.
Insert the right needle tip through the back loop of the new stitch, from back to front, and purl. I have found that this method leaves a hole with some highly-twisted yarns. If this happens, purl this stitch through the front loop instead of the back loop.