One of the positions is labeled CRANK the other IGNITION START. If you select crank then the engine is just motoring and won't start. This can be used to cool it and on turbofan engines it is used to wash the engine without risking people getting sucked in.
The other position is used for engine start. Let the core rotation speed build up and then introduce fuel by moving the engine condition lever from cut off to low idle. You'll see a sudden increase in engine temperature (ITT) followed by an increase in rotation speed. Once stabilized move the start switch back to neutral.