South Park famously annoyed the world by triggering Echo and Google Home devices with familiar wake words. When Amazon’s at the wheel, however, the company is able to ensure that Alexa stays quiet using a method called acoustic fingerprinting.

In the lead-up to the Super Bowl, the company’s offered a (relatively) easy-to-understand breakdown of why its celebrity-laden ads won’t wake up Alexa during the big game. With its own ads, the company adds a fingerprint of the audio, which is stored on-device.

Given the Echo’s storage limitations, additional fingerprints are stored in the cloud, where the assistant can cross-check things before waking. The system generally works pretty well, though complications can occur in, say, a noisy environment (what Super Bowl party has ever been noisy, though?) in which case a longer clip is required to do its job.

Things, naturally, get a bit trickier when Amazon isn’t producing the ad (as South Park fans can attest). In that case, the system cross-checks audio with different users.

“If the audio of a request matches that of requests from at least two other customers, we identify it as a media event,” the company explains. “We also check incoming audio against a small cache of fingerprints discovered on the fly (the cached fingerprints are averages of the fingerprints that were declared matches). The cache allows Alexa to continue to ignore spurious wake words even when they no longer occur simultaneously.”