When power goes to the coil, it stores up energy and when the current stops (points break or in your case, you turn off the car) it send the current through the secondary circuit, to the spark plugs.
Sounds like there is a fault in the triggering circuit, not cutting the current to the coil. Chick the wires to the distributor, check to see if the points are closed, it the cam rider is worn or if it is electronic, you might need a new unit.
jclay
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