TOPIC: AquaMeter Tachometer Wiring Question

Looking for some help understanding the "impulse" hook-up. I just acquired this tach and I'm trying to understand how the impulse option is set. It will be used with a 1969 Mercury 35HP 350. If I'm thinking correctly I'll have 2 impulses per revolution with this twin cylinder 2 cycle motor.

I've attached a picture of the back of the tach.

Thanks - Mike