Here it is, in black and white:
9-15 Pit Procedures During Race
M. The fuel handler must be in control of the fuel can at all times when fuel is being added to the car. The fuel handler will not be permitted to perform any adjustments or other pit stop procedures while the fuel can coupler is engaged with the car-mounted adapter.
Translation: So long as the gas man is pouring gas into the car, he is prohibited from doing anything else during the pit stop (In past, the gas man has helped make adjustments or stuck wrenchs in wedge-adjustment holes. This role expanded when catch-can position was eliminated).