Purling question

I was with a friend who was knitting something in the round last week, and she was doing something where she purled in the back of the loop. She said it makes the knitting tighter. I didn’t think to ask her to show me. Does anyone know what she’s doing?

There is a stitch where you purl into the back of a loop. It does twist it, and thus makes it tighter. I’ve never heard of anyone doing it randomly, but then again, I’m big on following directions. It’s also not something exclusive to knitting in the round.