SelfSync enables rapid, robust initiation of a gesture interface using synchronized movement of different body parts. SelfSync is the gestural equivalent of a hotword such as OK-Google in a speech interface and is enabled by the increasing trend where a user wears two or more wearables, such as a smartwatch, wireless earbuds, or a smartphone. In a user study comparing five potential SelfSync gestures in isolation, our system averages 96%, 98% and 88% for user dependent, user adapted, and user independent accuracy, respectively. For when the user has a phone in a pocket and a smartwatch, we suggest twisting the hand about the wrist while moving the leg with the phone in synchrony left and right. When the user has a head worn device and a smartwatch, we suggest twisting the hand while twisting the head left and right.