Just a simple P-wave detector...

"The appliance sends alerts once it detects primary waves, or the first waves of an earthquake that do not cause major rattling but travel faster than the secondary waves that are responsible for the actual shaking."

P waves travel faster than the destructive S waves. The further you are from the epicenter, the more the two have spread out in distance, so the further you are from the quake, the more warning time you have. The problem is, when you are far enough to get that 20 second warning, you are also likely far enough away to not worry about any serious problems - just sit back and enjoy the ride.

In the zone where serious damage occurs you'll only have a few seconds warning, if any.

Brian