This looks like a new UI shortcut for IFTTT.
It’s a separate app, the Do app, that has tiles. The “button” is a tile inside that app.
So right now it’s just a quick way to manually access an IFTTT recipe. (With the usual possible 15 minute lag).
My guess is this is IFTTT getting ready to compete with HomeKit, maybe even with some eventual local rules processing on the phone (hence the separate App). We’ll see. Even if it’s just UI to the existing IFTTT cloud, it’s a nicer interface for frequently used recipes.
You could have done this before inside the ST app with a virtual button, but the Notification Center option could be cool.