Here's Shell's site on this
Nitrogen itself will not "clean up gunk." However, that's not quite what they're saying: "Every drop of Shell Gasolines contain a Nitrogen Enriched Cleaning System that actively seeks and destroys engine gunk. Think of this cleaning system as a powerful soap, used to clean up grease from a tool. The Shell Nitrogen Enriched Cleaning System acts like this soap, attaching itself to the carbon deposits and dissolving them
And: "The Shell Nitrogen Enriched Cleaning System contains a "carrier fluid" that acts like water, washing away both the cleaning agents and the engine gunk into the cylinder, where gunk is destroyed as part of the combustion process. In addition, The Nitrogen Enriched Cleaning System acts like a barrier to keep engines clean and protect them from performance-robbing gunk better than before. Try Shell Nitrogen Enriched Gasolines and experience the benefits for yourself
Bottom line: It's simply detergents and protectants, same as always, though these days they're probably signficantly better than they were twenty years ago.