Ron asks:

"Retarding the spark timing helps to prevent detonation. If turning off one mag has the same effect as retarding the timing (as you stated above), wouldn't running on one mag help to prevent detonation rather than contribute to it?"

People tend to think of the ignition / combustion event as instantaneous but it is not. It is a process that takes time and involves a flame front that propagates across the combustion chamber.  If we define the combustion process starting at the crank angle when an arc is established across the electrodes of the plug and finishing at the crank angle when 95% of the fuel is burned then we can calculate the crank angle that is half way between these points, call it the median. If the combustion process takes longer then the median occurs later, retarding the overall combustion process even though the static ignition timing has not changed.

Generally, detonation seems to be a function of heat, radiation and TIME. Firing two plugs reduces the time and starting the process later reduces the heat and radiation.

During runup, turning off one ignition source has the same effect on RPM as retarding the timing in this case but the static timing remains the same.

Regards
Brent Regan