FYI, its not so much as controlling it as the MSD works completely differently and its not applicable.
In a point or basic electronic inductive ignition the dwell (or time that current is flowing through the coil) is approx 30 degrees. On an 8 cylinder engine for each rotation of the distributor current is flowing 240 degrees of the 360 degrees of rotation or 66% of the time. Current flowing that much of the time will over heat the coil and cause it to fail. The ballast resistor limits the current flow to protect the coil.
An MSD box charges a capacitor and discharges it across the coil. The discharge happens in milli seconds but at a very high voltage (400 +). Becuase it happens so fast the coil never has a chance to heat up enough to cause problems. Because it happens so fast that there is enough time to do it three times per ignition event below 3000 rpm.