when I stage the car I have the horn select switch to set to control the line lock/two step when I push the rimblow. When I stage the car, I set the line lock switch to the position, such that when I activate the rim blow and the horn select switch is set to control the line lock/two step position, then it will send a signal to the MSD box that has a two step rev limiter. So basically, i'm activating the line lock solenoid simultaneously (but it's not doing anything since I didn't put any pressure in the brakes). however, it will also set the rev limiter to my two step limit (e.g. 5000 RPM instead of the normal 6500 when the two step input is active). Doing this allows my to have the throttle wide open, prior to launch. This results in FULL air flow through the motor (e.g. high speed airflow through the manifold which should result in normal operation of a carb with the boosters). This alleviates issues with a carb having to rely on the accelerator pump circuit and manifold/air flow issues that can occur and low rpm.
If you aren't drag racing then there is no need for a two step or really a line lock.