Purl 2 together through the back loop, or p2tog tbl, is a rather counterintuitive way to decrease stitches on the purl side, but once you understand that it's going to feel awkward, it gets a little easier.
To make the stitch, you need to approach the two stitches you want to purl together both from the back and from left to right rather than from right to left. This is really the only way to have the needle in the right position to get the working yarn in order to form the stitch.
It's often been said about this tricky technique that if you feel like you're doing it wrong, you're doing it right. But after a little practice it is not that tricky, and it's an interesting way to decrease on the purl side.
You probably won't run into this stitch very often, but it is involved, for example, in the Cat's Paw or Crown of Glory pattern.