What things were contacting each other?
Contacts in the electrical switch. (I think anyhow.) Now someone can come along and say otherwise. 
I’m pretty sure spinning the blade by hand gave the engine cylinders compression and the pilot flipped a switch for ignition. Igniting the compressed gas/air mixture in the cylinder.
The reason for yelling like this ?
The ground crew had to prime the engine, by turning it over. This does all sorts of things such as moving oil around and getting the fuel up to the motor.
it is vital that the “contact” (ignition) is off at this time, so that the motor won’t start… because if the motor starts during priming, someone could get chopped up by the propeller… or shocked by the high voltage on the spark plug leads.
Once the motor is primed, the ground crew will then notify the pilot that they are ready to start, and for the purpose of having a word that won’t be confused
(Switch ? there’s lots of switches… on, off ? maybe they are talking about air or fuel ? Magneto ? (the generator to make the electricity to run the spark) ? This could be confused with other magnetos such as to run lights or fuel pump or something.) So they had the unique word “contact”. Pilot yells it back to ground crew to be sure ground crew know that the motor could start if they turn it over…
I once had a car that started the same way. I would thread the starting handle into the front of the engine and turn it over a few times and then move it to just before a compression point. Then I went round and switched the ignition on, yelled “Contact” gave the handle a sharp pull and we were away.
And in the BBC series Wings, a Great War pilot (2nd Lt Gaylion) forgot this vital step owing to battle fatigue, a mechanic turned over the prop while the ignition was live, and a moment later the engine fired and both his hands were severed at the wrist. :eek:
Footnote: Wikipedia mentions that the ignition switch was faulty so Gaylion was not necessarily to blame. Can’t expect me to get all the details right after 37 years…!
Hillman Imp?
As **islider **said.
The two magic terms where/are “Switch off” meaning ignition off, and “Contact” meaning ignition on. The words were deliberately chosen to sound very different to preclude confusion. Screwing that up has maimed or killed a lot of ground crew over the years.
The ritual has the ground crewman yell what he wants, then the pilot yell the same back after complying. Which ritual extends today into most pilot-to-pilot coordination in the cockpit, and all ATC controller-to-pilot-to-controller radio work.
Probably a Renault 4 or Citroen. They kept them up to the eighties!