When you crawl under the truck to install the jumper, you will either see the switch on the end of the slave cylinder assembly portion of the Air Pack, or else it will be on the side towards the frame of the same assembly.
The hydraulic on is on the end and is activated by pressure in the brake system itself and since you can make the brakes work without air, it will work regardless of air pressure or not.
The air switch, as pointed out abive, located on the side of the slave clynider will only operate the brake lights when you have some air pressure in the system.
A jumper will turn either type of circuit on in terms of the brake lights.
The air operated switch is in my opinion a superior setup as their is no proximity of DOT 5, which is flammable, and electricity as there is in the hydraulic activated switch. Even so, there are thousands of deuces out there with hydraulic brake switches and they manage just fine.
I changed mine over, but that was my choice and I did it when I was rebuilding the airpack and it was pretty easy to make the change when I had the airpack out of the truck.
Hope this is helpful
RL