SynchroWatch is a one-handed interaction technique for smartwatches that uses rhythmic correlation between a user's thumb movement and on-screen blinking controls. Our technique uses magnetic sensing to track the synchronous extension and reposition of the thumb, augmented with a passive magnetic ring. The system measures the relative changes in the magnetic field induced by the required thumb movement and uses a time-shifted correlation approach with a reference waveform for detection of synchrony. We evaluated the technique during three distraction tasks with varying degrees of hand and finger movement: active walking, browsing on a computer, and relaxing while watching online videos. Our initial offline results suggest that intentional synchronous gestures can be distinguished from other movement. A second evaluation using a live implementation of the system running on a smartwatch suggests that this technique is viable for gestures used to respond to notifications or issue commands. Finally, we present three demonstration applications that highlight the technique running in real-time on the smartwatch.