The idea is the IR lighting would be placed up front beside, below, or above the projector screen, and interfere with the visual recording device to ruin the captures. In my experience digital camera sensors are sensitive to IR light in low light environments.

I've never tried in a movie theater :p but have noticed this effect by pointing my phone camera at an IR television remote and pressing some buttons. The IR LED lights up and is only visible to the camera and not the naked eye.

Anyhow, I'm more just curious why studios / cinemas don't take relatively simple measures like this to prevent pirate recordings from being created. Perhaps the IR would be too easy to circumvent via a physical lens filter.