HU/Sub/Speaker Splitter

My buddy was saying that the stereo shop that installed his stereo used a "splitter" to send a seperate signal to his am so he doesn't risk blowing his head unit or factory speakers.

What is he talking about?!?

I've tried Google, for hours, and come up with nothing.

Nevermind. I called the place he got his stereo installed at, and the guy remembered the car. He aid it was just a Line Out Converter.