What does the term “hot mag” mean? How can I test for it?
“Hot mag” means that your ignition switch is not successfully killing the ignition on one or both magnetos. As a result, if someone moves the prop and conditions are just right, the engine can fire. Hot mags are unpredictable, since a wire lead can break at any time, which is a key reason for treating the prop as a loaded gun.
Very often the first indication of a hot mag is lack of mag drop during the mag check. No drop at all usually means that the magneto is continuing to fire, despite being turned off at the ignition switch. For example, when your ignition key is on “L”, the switch is grounding the right mag (you are running only on the left mag). If the lead to the right mag becomes open, the switch won’t ground it, and you’ll get no mag drop (since both mags are still firing). Unlike an auto ignition, your mag switch does not send power to the mags. The mags are self-generating. Unless something prevents them from doing so, they will fire any time that they rotate fast enough.
Your ignition switch is used to ground the “P-lead”, thereby shorting the mag so that it cannot generate spark. If the P-lead connection becomes open (often due to a broken lead at the mag-end terminal), or the ignition switch fails open, your prop can be an serious accident waiting to happen.
Please read the information at this link, for a more in-depth explanation: