"Dry" nitrous system

In a "Dry" nitrous system, extra fuel required is introduced through the fuel injectors, keeping the upper intake dry of fuel. This property is what gives the "Dry" system its name. Fuel flow can be increased either by increasing the pressure in the fuel injection system, or by modifying the vehicles' computer to increase the time the fuel injectors remain open during the engine cycle. This is typically done by spraying nitrous past the MAF sensor (Mass Air Flow), which then sends a signal to the vehicles computer telling it that it sees colder denser air, and that more fuel is needed. This is typically not an exact method of adding fuel. Once additional fuel has been introduced, it can burn with the extra oxygen provided by the Nitrous, providing additional power.
http://en.wikipedia.org/wiki/Nitrous