I have recently seen the same thing, and I think he is describing a situation like what breaks the nosecones off starters on GM4.3 powered pre CARB trucks, LPG powered. it is from a spark generated when the coil's field collapses as the key is moved, with the piston stopped with compression and fuel in the cylinder.
If it was me, I think I would replace the igniter in the distributor, do a complete tuneup check the wiring and the key switch itself, and this time be sure to use the heat sink compound and clean off the old heat sink compound.