פירוש על זבחים 11:6

פוחתו (hollows it out) – for a metal utensil is not pure from its defilement with an mere incision, but rather with a large incision. But however, after it is hollowed out, they strike it with a mallet and fasten it in order that the name of a utensil is upon it. For at the time of scouring, it must be a utensil.

If a copper vessel was carried outside the hangings, it re-enters and is scoured and rinsed in a holy place.
If it was defiled outside the hangings, it must be broken through, then it re-enters and is scoured and rinsed in a holy place.
This mishnah teaches the exact same law as was found in yesterday’s mishnah, except that today’s mishnah refers to a copper vessel. Since the law is the same, I refer the reader to yesterday’s mishnah for more commentary.
Section two: In order to purify the copper vessel, he must make a large hole in it, for metal vessels become pure only if large holes are put into them.