Final answer:
The N2 indication on an engine's gauge should start to increase a few seconds after pressing the engine start button, depending on the type of engine.
Step-by-step explanation:
In the context of engine start-up, an N2 indication refers to the rotation speed of the low-pressure compressor, which helps compress air for combustion in the engine.
Seconds after pressing the engine start button, the N2 indication on the engine's gauge should start to increase. The exact time will depend on the type of engine being used.
For example, in jet engines, the N2 indication typically starts to rise within 2-7 seconds after pressing the start button.