I use them when if I break my chain on a ride... there's no need to replace the whole chain until it's worn out. I had 3 power links on my chain from 2 years ago. Last year had 2. My new one has one... for now anyway.
Trail: XXX Riders: Pinkbike Product Picks Description: Small and mighty, Bionicon's C-Guide is a well-executed compromise for single and double-chainring riders who want quiet, effective chain control. The derailleur cable housing can be routed through the middle of the C-Guide's tubular plastic hinge pin, so it doubles as a cable guide as well.
Camera: Nikon COOLPIX P5100 Shutter Speed: 1/350 Aperture: f/7.1 Focal Length: 24.5 mm ISO Speed: 64 Date: 2012:02:16 Time: 15:11:09
http://bikotic.com/2011/09/13/macca-dangler-mk-2-home-made-chain-device/
-CF-