The sync gesture is how the Myo armband listens to your arm. When you sync, it figures out its orientation in space, its position on your arm, and which arm it's on.
In the video below, Phil shows how to perform the sync gesture and explains the importance of a good sync.
How to do it
Make sure you're wearing Myo with the USB port facing your wrist. Gently flex your wrist away from your body. Myo will begin to vibrate when it recognizes this gesture. Hold this gesture for a few seconds until Myo stops vibrating.
You will know you performed the sync gesture successfully when the Thalmic Labs logo LED on the armband stops pulsing. If it needs to warm up, you will see it blink along with an notification next to the gesture indicator window in Myo Connect. Once Myo is fully warmed up and synced, you will feel three distinct vibrations.
The sync gesture is stored on the Myo armband itself. If you are using multiple devices, disconnecting and reconnecting Myo will not lose your sync. As long as Myo stays on your arm, you can have the same sync with every device you connect to.