That swatter are made to deliver shock to the paste core, or something like that. It caused yukkuri to faint instantly. Good if you need quick and clean method to silence a yukkuri without killing or damaging them.
But trainer need them to stay conscious. Can't teach a student who keep losing consciousness with a single slap. Abyuser want to see them suffering. Knocked out yukkuri are no fun and while waking them up creatively is a challenge, doing it repeatedly out of necessities can be a killjoy.