Toby has it basically right. For any particular AS rule, if you can show
it was legal either the current year, or the previous year, then it's legal.
So if SCCA makes a rule that is a super advantage to you, and you
immediately change to take advantage of it, you're legal. If SCCA makes
a rule that causes you to have to add 200 lbs, then you can stay at
any weight you want that was legal within the last 2 years.
So, it allows a driver to update as fast as they want, but doesn't force
anyone to update until something has been in place for at least 2 years.
This, in my opinion, is much more friendly to a racer who may not have the time
or inclination to keep completely up-to-date with how fast SCCA rules may change.
BTW, the SCCA runoffs crap in the tech tent & COA this year was a travesty. Basically
to me, it sounds as though the COA overruled the tech folks on the whole
valve lift rule, making over lift limit competitors legal :-(. Thankfully, they showed some
sense in the carb screw ruling.