Split and Idle Emissions
 

This controls the spark for "emisson" purposes...isn't this why they have trialing plugs? Or is it just for MPG?

Could a lower or higher split lower the HC and CO in emission testing?

It's to aid in a complete burn in combustion (flame propagation).

This improves power/mpg/emissions

HC is mostly unburnt fuel IIRC therefore it could be used to alter the HC content. I think the more HC the less CO you have but i'm not sure.