How do they put the slit into those pre-packaged hot-dog buns?????

Like, they come in packs of 6 or 12 and theyre stuck together in sets of 3, so i can understand how 1 of them gets sliced, but how does the middle one and side one get sliced...cause theyre all already pre-cut when you buy'em.

WTF!??????

...your thoughts please.