HDMI uses HDCP to verify the receiver (TV) is authorized to receive DRM'd content.
This and there's a signal from the mothership to the cable box that periodically needs to be answered. If it doesn't get the answer it's looking for, it disables things like Pay Per View. That said, I don't believe they ever rolled it out to widespread usage on non premium content.
Also, right around the time they started broadcasting over the air TV using HD signals (requiring a converter or new antenna if your TV couldn't handle them - somewhere around 2008ish) a lot of TVs could also scan for channels using fractional numbering - ie 7.1, 7.2 etc. Most providers put their video on demand and PPV stuff on a specific channel that was then broadcast over that while your cable box was tuned to pick it up. If you scanned for channels periodically, you could watch all kind of content that someone else ordered and was watching.
At first, the picture was fine - looked just like you had ordered it. Within a year or two however, the DRM kicked in and you would get static/fuzz/censored/distorted display when played at normal speed, but you could see what was happening without distortion when someone used the fastforward or rewind features.
I believe it was after that point when they introduced the signal from the mothership to ensure you weren't picking up channels you shouldn't - want to say it was Q tuning or something along those lines. It's been a while.
