SAP also stands for the Secondary Audio Protocol, a TV audio encoding standard for the visually impared. When tuned into an SAP-enabled channel, narration as to what the characters are doing will be read during the interim in the dialogue. This allows visually impared people to listen to the television as if it were a narrated storybook. For normal viewers, it does not affect the actually television viewing much, IMO.

My grandmother loves this, as her mysteries on PBS are almost all encoded in this. You need a television capable of picking up these signals on the channel, to hear it. Oftentimes it reads what you would see in the closed captioning, ironically.