Just for the history books, there originally were two forms of forced unmounts. The gentle force (-f) and the brute force (-F) unmount. The -f unmount flushes out all the dirty buffers so that when the unmount completes no data is lost and the filesystem is in a consistent state. The -F unmount invalidates and discards all the dirty buffers without attempting to do any I/O on them. The result is lost data and a possibly inconsistent filesystem. But it will get the job done even if the disk has died or the server has gone away.

For reasons that I never tracked down, the -F unmount option was never incorporated into FreeBSD when they did the merge from 4.4BSD-Lite II, so that functionality never made it into the system. It is actually much easier to do than unmount -f since you just walk through and set B_INVAL and B_ERROR on all the dirty buffers for that filesystem. The problem with unmount -f is that it will hang if the server is gone since it will insist on pushing back all the dirty buffers.