For DE events, passing is usually at the direction of the car being passed and can be on either side at their discretion. It is usually recommended that the slower car stay ON LINE, as LD said, and the passing car must go off-line to make the pass. Sometimes a club will specify passing on one side only if the track lends itself to that or if all of the drivers are green enough to need that level of restriction, but it's definitely not the norm.

I'm pretty sure Craig was in NASA's advanced group, which allows open passing. It's almost like a race, only more "gentlemanly". Passing happens whenever and wherever the passing driver feels it is safe. (sometimes they'll limit it to straights) It's a whole new world when you hit that level. I still remember the first time I ran CMP w/ NASA and they put me in that group... having the liberty to choose to make a safe pass in a turn was gratifying.

(of course, now I run LeMons races and make passes with 2 wheels in the grass at my discretion... even MORE gratifying!)