Motorcycle Chain - O-Ring or X-Ring
X-ring motorcycle chains have a seal that has an X shaped cross section rather than circular and therefore, as the theory goes, it has a small contact patch with the chain and produces less friction and wear.
Think of the number of links in a motorcycle chain and you can see that if this is true then the combined reduction of friction of hundreds of X-ring seals much add up to some performance gain...
Honestly I never experienced any performance difference between a O-ring or X-ring chain. But some people report that they actual feel a performance gain while using an X-ring motorcycle chain. Most of the time hard evidence in the form of dyno data is not available.