Engine revolution sensor?

[Booster]

Hey guys. I've got a 1990 GQ RB30. I am in the process of connecting my tacho up after installing an MSD ignition. I've got the wiring diagram but cannot seem to understand what the engine revolution sensor is or what it does or where it is. On the diagram the -pulse wire from the dizzy splits, one going to the -wire on the coil and the other through a resistor to the engine revolution sensor then to the tacho. I thought the tacho should just get a negative pulse from the -coil wire but as my msd is using the -pulse wire from the original -coil I've got to connect it up to the tacho out from the MSD box. My other option was to splice the -pulse wire with the MSD pickup wire but don't know if that will have any bad effects on the MSD unit signal. Basically I want to use the original wiring from the dizzy to the MSD tach output so I don't have to take my dash apart.

Anyone help.