The RAF method to test mags was this:
At 800rpm, idle:
Drop no stop (Right), BOTH, Drop no stop (Left), Drop and Stop (Off), BOTH.
The last part was done very briefly so that the engine was kept running.
RAF Bulldog, rotary mag switch, 200 bhp Lycoming.
The idle cut-off is pulled to purge fuel from the cylinders so that an ungrounded magneto by itself shouldn't result in a propellor bite if someone turned the motor over compression.
However, as always, the manufacturer's method in the POH is the correct way to do it.