WoCase Pendant Necklace (Classic Style) for Fitbit Flex (Best Gift for Fitbit Flex User) Activity and Sleep Tracker Wristband Band Bracelet (One Size, Fits Most Wrist) Lost-proof Design and Stylish. Best pendant (patent pending) for Fitbit Flex activity and sleep tracker. To open the clasp, push the two pices of the clasp toward each other until there is no gap between them, then twist the two pieces in the opposite direction (While pushing them toward each other.