As mentioned, it connects the house battery and the chassis batteries together like a jumper cable would.
It doesn't actually start anything. If the engine or generator seem to be cranking slow, you would push and hold it on, for a minute or so, and then while still holding it on, attempt to start the slow cranking engine.
As a side note, if it doesn't work, you would want it fixed. In most cases it is also the same solenoid that automatically closes to charge your house battery while the engine is running.