You might have heard of Yo. It’s the app that lets you buzz a friend’s phone with a "YO" push notification. If you swipe on it, you’re brought into the app where you can proceed to send a Yo back to your friend. It sounds a little ridiculous, and a Yo war may ensue, at which point you might delete the app. But hold on a minute — beneath all the Yos, there’s the seed of a very interesting idea. Yo presents an almost absurdly simple mechanism to quickly subscribe and unsubscribe from push notifications. There aren’t many services taking advantage of it yet — for now, you can only Yo WORLDCUP to turn on notifications for when there’s a goal — but in the future, you might receive a Yo from a restaurant to say your table’s ready, or from an airline to say that your flight’s boarding, or from your weather channel that there’s a flash flood warning.