Using an articulation tube, physically-limited gamers with special needs could "sip and puff" with the HFC instead of pushing the A and B buttons on a traditional controller. Sipping and puffing with more force would translate into hitting Select and Start. The button modes and pressure settings were adjustable with several knobs and switches on the device's control panel.
Because it was never sold in stores and was only available for purchase through Nintendo's customer service line, the HFC is one of the most rare NES accessories out there. An unused bundle that showed up on eBay recently commanded a Buy It Now price of $379.99! Head past the post break for some old magazine scans advertising Nintendo's Hands Free Controller.
NES Journal (Click resized scan for larger image):
Nintendo Fun Club, prototype (Click resized scan for larger image):
[Via Gamesniped, Digital Press Forum, and Nintendo Player]