The MSD "ready to run" is a conventional ignition, similar to points, Mopar breakerless, or pertronix, or GM HEI. That is, the trigger switches the coil NEG to ground, and the coil + is fed DC power from the electrical system.
MSD CDI (or any other CD) DOES NOT work that way. In the case of a CDI, or MSD 6, etc, the coil becomes a transformer, and the ignition "when it fires" discharges a big pulse into the coil. This is similar to an electronic photoflash, which also uses a big capacitor, hence the name Capacitor Discharge Ignition.