A stim is a simulator.
This plugs into the megasquirt ECU. It enables you to test the functions of the megasquirt, simulate engine loads & allows you to set the basic settings via the tuning software.
If all goes well. The engine should at least start once you have pre-configured the ECU with the stim.
That's the theory anyways
If you have bought a ready built unit it would have been set up using a stim.
