ignition coil wiring

[warnockm]

I have an Accel HEI Super Coil Ignition Coil and want to understand how it is wired. There is are two wires on the "input", the red says "BATT", battery obvious, and the yellow says "TACH". On the center is the high voltage output that goes to the distributor and spark plugs, and there are 4 bolts that are common ground.

I assume the BATT is constant 12v, but is the TACH and input or an output? Looks like the distributor creates the "pulse" used to create high voltages. does the yellow wire "Read" the pulse from the distributor, and this is where the tachometer can be connected? if so, why does it need both yellow and red?