Orval Fairbairn writes:

[...] all the P-lead break does is prevent you from shutting down
that mag. It is a good idea to pefrorm a "both off" check when
doing a mag check. That way, you will be able to find a broken
P-lead.

I've heard mechanics advising against doing this test casually. If
done at even moderate power, it can apparently cause backfiring, which
can in turn create induction system damage. I've seen a C172, one
cylinder of which was burned out shortly after such an episode, due to
extreme lean running resulting from induction cracks.

- FChE