The ignition key is a pretty good power cut off that is driver accessible. If you want something that kills all of the power then that gets a lot more difficult. Especially with EFI and/or other electronic systems that don't want to be shut off hot.

I'd be interested to see how various folks are wiring up the EFI computers in race cars. Holley wants the ECU connected directly to the battery without a cutoff switch. I don't know if NHRA allows that or not. I suppose it depends on the tech person.

The car can be shut off by turning off the switched 12v to the ECU but the ECU is still connected to the BAT so there are still hot wires inside the car.

NHRA has never defined the master cutoff very well. The rules just say it should turn off the car but they don't say how much stuff can still be connected to battery power when the engine stops running.