The mag is "HOT" when the "P" lead is open, i.e. switch "OFF". Turn the switch on to ground and "kill" the mag.
Typically a SPST switch is installed upside-down so that the mag is "HOT" with the switch in the up position.
Mel Asberry..DAR since last century
A&P/EAA Tech Counselor/Flight Advisor
Specializing in Amateur-Built and Light-Sport Aircraft
North Texas (8TA5)
RV-6 Flying since 1993, 172hp O-320, 3-Blade Catto (since 2003)
Legend Cub purchased 12/2017
FRIEND of the RV-1
Eagle's Nest Mentor