How does a DVD recorder detect DRM on an Analog video signal?

I finally started using a DVD component unit (usually I do all my recording via PC) and was very surprised when I played an Amazon download and hit record on the recorder and it stopped and displayed “COPYRIGHTED CONTENT - CANNOT RECORD” or words to that effect.

I’m using the S-video input which is analog (only digital video-in would be HDMI, right?) so how does the device detect copy protection? I’m not asking for instructions on how to defeat it, I just don’t understand how they’ve plugged the ‘analog hole’ for video.

Analog Copy Protection. Colloquially known as “Macrovision Protection.”

Quoting from that link:

ETA: there may be further methods; ACP is the first mentioned…