On a photoshoot, the person responsible for the digital equipment such as the camera and the computer to which the camera or digital back is tethered. They import images, regulate the white balance and exposure, oversee proper color management for monitors and printers, alert the photographer to adjust the camera due to a change in lighting conditions if it's necessary, and alert the photographer of focusing issues. The most common setup is a Mac Pro with Mac OSX running Capture One, but Adobe Lightroom and Hasselblad Phocus are also used when Capture One doesn't support tethered capture with the camera or digital back used.
More frequently, however, they deal with Capture One's annoying habit of crashing every so often, troubleshoot people's iPhone issues between shots, quietly change the camera settings on their own from the computer if the photographer bumps something by accident, wipe art directors' fingerprints from the monitors regularly, work many hours after the shoot when the art director suddenly decides s/he needs full-res JPGs instead of 1200x1800px JPGs even though the minute details are negligible for something that will be retouched and then printed at 1"x1.5" in a newspaper flyer that will be thrown away by the majority of people who receive it.
None. The assistant does it, but the digitech points out the bulb was fine; they just didn't flip the switch.