I was learning Pesachim, and on 17b I saw Tosafos say the following:
דבתוך הכלי ודאי אסור לטבול מדאורייתא
It is definitely biblically forbidden to immerse something within a vessel (as opposed to a mikveh)
I was very surprised by this phraseology. I would have said instead that immersing something in a vessel filled with water doesn't work, not that it is forbidden. Why does Tosafos say it is forbidden? Is this just an interesting way of saying it doesn't work? Or it's referring to the fact that it will lead to forbidden things, such as someone spiritually impure eating kodshim or entering the Temple? Or is there a bona fide prohibition. If so, what is it?
I did a search on Otzar HaChochmah using the wording of Tosafos and found many Rishonim use this expression. This enlarges my question.