Passed along by Alex (thanks!).
Turtlenecking is a fetish, or paraphilia. It's related to wool fetish (i.e., woolies). The images in this post are from a tumblr featuring turtlenecking: link (You can also check out a dedicated wool and bondage website with photos here: link)
The tumblr site includes a good description of turtlenecking and why it's such a turn-on for some people:
Turtlenecking or facewank is a fetish about rubbing face with collar of turtleneck sweater. It is a game when head plays a role of dick and turtleneck plays role of foreskin. “Condom” can be used - nylon stocking or pantyhose over ther head to make frictions easyer and protect face. During frictions wool is producing smell which is important part of play. Also there are analog of cum - if turtlenecked person asked to keep liquid in mouth during wanking or if ballgag used which produces saliva drooling. Originaly fetish appeared at website woolbondage.com. At the “students party” ordinary turtleneck sweaters are in use. In advanced fetish games players can use special collars with rubber bands under armpits for moving collar down. There are self-turtlenecking and turtlenecking by partner. Practicers believe that stimulating of head which has many erogenous zones can have as a result sensitive emotions up to orgasm.
