An inverted roll is simply a roll in which the first note of each double stroke is on the "&", rather than the downbeat. So, instead of: RRLL RRLL.... you'd play RLLR RLLR
Kenny, however, leaves off that single note on the downbeat and simply starts straight in with the double strokes on the & of 1, which gives the whole thing a nice kick. It's worth trying it in different parts of the bar, as well as different parts of the phrase, though it works particularly well on the & of 1, and the & of 2.
I'm toying with the idea of tying the blog into YouTube a little bit more, so there is a video down below demonstrating this idea.