Two-step rev limiters have two separate RPMs for which a rev limit is set, depending on certain conditions. Typically the first rev limiter is used when the car is stationary, as launch control.
The second rev limiter is the redline (or slightly past) there to ensure you don't damage the engine by over-revving. Rev limiters work by either limiting fuel, or limiting spark. You can run a lean A/F mixture, or cut fuel entirely. Likewise, you can retard spark timing, or kill the spark entirely, but this means unburnt fuel will exit the exhaust, thus leading to pops and crackles as the unburnt fuel ignites with the outside air.
Two step is not by design an anti-lag system, but by significantly retarding the timing, it can act very similarly, and achieve the same goal, though only for launches. Once the vehicle is moving, the system will not attempt to maintain boost during shifts, as traditional anti-lag systems would.