How does "Increases bonus experience" on items work?

Leoric's Signet, Cain's Fate (set bonus) both give bonuses to experience worded like this:

Increases Bonus Experience by X%

I don't have either of these to test it out, but I'm curious to see if they are worth the investment.

Do these items work like a Ruby in your helm socket, increasing the experience for monster kills by 30%?

Or is the affix read literally to mean it increases only your bonus experience? I.e., with a 30% ring and you have +100 xp per kill, the effect increases that to +130 xp per kill.

If the latter, what qualifies as bonus experience? Just +XP per kill? What about NV stacks? What about a ruby in your helm?

Both seem like legal interpretations of the wording, but it is a huge difference in the attractiveness of the items.

Can anyone shed some light on exactly how Leoric's Signet and the Cain's Fate set bonus work?