That is what you do when the wrench won't grip them any more and you
can't get in with a vise grip or a pipe wrench. There are actually a
couple chisel methods. The first involves using the chisel to turn
the nut by hammering it in a tangential direction. If that doesn't
work you use the chisel to split the nut by hammering it axially.
Usually you only have to split it on one side to get it off but
sometimes you have to do it on both sides.

Note that injudicious use of a chisel, either sharp or dull, can increase
problems rather than eliminate them. A bigger hammer works better but
can also cause more consequential damage. Chisel work is the responsibility
of the person with the hammer in hand. NO responsibility is assumed by
others, regardless of any assumptions made by the chiseler.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma