Once a second, the encoder's lowest variant is compared with every slate. Difference is the average change across every pixel (0% = identical). A slate at or under its threshold starts an ad break of its duration, unless a break is already running or announced. After any break ends, detection waits out the cooldown, so a slate still on screen doesn't start another break straight away. The comparison reads the encoder's output live, so a slate is caught a second or two after it goes to air (ahead of the preview, which plays a few seconds behind).