Twitch is really clever about it.
Because you're watching a live stream, they know you're less likely to pause or look away, and then because they're using HLS, they just literally have the next chunk of video load from their ad servers instead of their video servers.
The most clever part? This is done at a point before your computer, so you can't distinguish which is which as to you, it all looks like its coming from the same source.
Now, perhaps they still give you some information about the stream that they're in a game of wackamole with block detectors over, maybe for tracking purposes or whatever, but its pretty clever, like dastardly clever but clever none the less.