MSD's Website says this:
The RPM Activated Switch, PN 8950, has two activation wires; one to ground a circuit and the other to open a circuit. Simply plug in an rpm module and wire the Switch to the circuit you want to activate. When the engine rpm reaches that amount, the circuit is activated and will remain on until the rpm falls below that amount.
-for the relay-
The SPST relay is rated at 30 amps with an input voltage of 12 volts DC. This relay is ideal for use with the MSD RPM Activated Switch PN 8950 when 12 volts needs to be switched to a high current device such as an air shifter, or nitrous solenoid.