Resistor Used With Master Battery Cut Off Switches?

[Atari Boy 11]

I see that some master battery cut off switches come with a resistor such as this one HERE .

I have a cut off switch but not the resistor, what do I need to buy (spec etc), what does it do (some adverts on ebay say it will protect my alternator) and where should I put it?

 

Thanks

 

Jonny

[SPGTi 2]