Mini-nuke in mouth... because of course.

I always picture that self destruct CLICK at the end sounds like Killer Queen's switch CLICK.

The narrator said:

In case of emergency, Marisa has prepared a mini-hakkero in her tooth for self-destruction.

And since Rinnosuke made the hakkero: "Kourin King! Daisan no bakudan... BAITSA DASTO!"