What are those cords that many players are wearing around their necks?

they are healing necklaces. i can't really explain it more than that cause i'm not sure that i quite understand how they work. but here's an example that i found when searching... http://cgi.ebay.com/PHITEN-TITANIUM-18-BLACK-WHITE-HEALING-NECKLACE_W0QQitemZ170141426666QQihZ007QQcategoryZ101970QQcmdZViewItem