This module exploits an arbitrary command execution vulnerability in tnftp's handling of the resolved output filename - called "savefile" in the source - from a requested resource. If tnftp is executed without the -o command-line option, it will resolve the output filename from the last component of the requested resource. If the output filename begins with a "|" character, tnftp will pass the fetched resource's output to the command directly following the "|" character through the use of the popen() function.
To display the available options, load the module within the Metasploit console and run the commands 'show options' or 'show advanced':
msf > use auxiliary/server/tnftp_savefile
msf auxiliary(tnftp_savefile) > show actions
...actions...
msf auxiliary(tnftp_savefile) > set ACTION < action-name >
msf auxiliary(tnftp_savefile) > show options
...show and set options...
msf auxiliary(tnftp_savefile) > run
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