In a nutshell you can fill your oil pan right before a race with nitrous and during the next pass on the motor the nitrous will dispense into the cylinders via the oil passages. This will not give huge gains but pretty good ones.

There is a lot of testing you have to do with this method to get it right. First you have to see how long it takes your particular motor to come up to the temperature to dispense the nitrous you have to find the correct method of getting the juice into the oil pan and finally and most importantly you have to tune the motor correctly.

This was supposedly the way the Dodge Boys were cheating a few years ago in the NHRA Pro Stock class.