VOP_INACTIVE(9): reclaiming space for open but deleted files?

[Alan Somers]

VOP_INACTIVE(9) says that the vop can "be used to reclaim space for
‘open but deleted’ files".  What does that mean?  How can you reclaim
space for open files?  I assume it's just a mistake, but I want to
check before I fix the man page.  SVN archaeology shows that the line
has been present since a mass import of man pages in 1997.
-Alan