Not sure how it's communicating with the phone.

Maybe it's the tech from that company that uses songbird-like sounds to communicate back and forth?

How does that syncing thing actually work? Light flashes? Vibrations?

Light or Vibration were both of my guesses as well. I personally think that wireless is better. The new Fibit syncs over bluetooth 4.0 and it uses very little power while doing so.

Think it must use the magnetometer.

It's interesting that compatibility is iOS5+ and Android 4+

How is this going to charge, I wonder?

They could potentially do wireless inductive charging through the same coil they (hypothetically) use to do magnetic communication. That, or the top/bottom half of the device are isolated and act as the -/+ terminals (probably wouldn't be good for the swimming application though).