I hear this term thrown around a lot in when it comes to rally cars. What is 'anti-lag'? Is it just nitrous used to spool the turbo?

 

In a rally car, not nitrous. I am sure they all have a few different approaches but a simple explanation of one would be as the ignition system is retarded, a lot, so when the spark ignites the fuel the exhaust valve is open or opening. Lots of fire helps spin the turbo extremely fast, but puts a lot of stress on everything.

 

Google is your friend.

http://www.rallycars.com/Cars/bangbang.html