What he’s saying is the spark plug holes inside the rotor housing are too large (much larger than apex seal width) so at peak compression and just prior to combustion, a percentage of compressed air/fuel mixture enters the spark plug hole and leaks ahead of the apex seal into the exhaust cycle, creating spontaneous combustion in the exhaust and higher exhaust temperature.  His theory is if the holes were slots, similar in width to an apex seal this would stop any leakage into the exhaust cycle – more complete combustion = higher efficiency – don’t put air fuel into the exhaust for spontaneous combustion.

Jeff