This command uses the Fuse Filesystem (FuseFS) to mount a directory at DIRECTORY that contains the content of all check-ins in the repository. The names of files are DIRECTORY/checkins/VERSION/PATH where DIRECTORY is the root of the mount, VERSION is any valid check-in name (examples: "trunk" or "tip" or a tag or any unique prefix of an artifact hash, etc) and PATH is the pathname of the file in the check-in. If DIRECTORY does not exist, then an attempt is made to create it.
The DIRECTORY/checkins directory is not searchable so one cannot do "ls DIRECTORY/checkins" to get a listing of all possible check-in names. There are countless variations on check-in names and it is impractical to list them all. But all other directories are searchable and so the "ls" command will work everywhere else in the fusefs file hierarchy.
The FuseFS typically only works on Linux, and then only on Linux systems that have the right kernel drivers and have installed the appropriate support libraries.
After stopping the "fossil fusefs" command, it might also be necessary to run "fusermount -u DIRECTORY" to reset the FuseFS before using it again.