Don't know the science behind it, but it makes a big difference. Light aircraft engines are mostly horzontal opposed "flat" fours and sixes, most designs dating back to the late 30's. They mostly use dual magnetos and dual sparkplugs. A pretakeoff check is to check the ignitions one at a time, looking for excessive RPM drop when going from single igntion to dual igniton, indicating a fouled plug or worse. Once was taking off one morning from a gravel bar alongside a river in Alaska. No good place to do a power runup, so just did a rough mag check while taxiing. Real slow takeoff and acceleration, had to turn upriver and not doing so good. Was running on one mag. Improved considerably when I switched the other mag in. Work swiftly; not hastily! My $0.02