In Unix/Linux, a removed file only disappears when the last file descriptor to it is gone. As long as the file /usr/bin/rm is still opened by a process (and it is, because it is running) it will not actually be deleted from disk from the perspective of that process.

This also why removing a log file that's actively being written to doesn't clear up filesystem space, and why it's more effective to truncate it instead. ( e.g. Run > /var/log/myhugeactivelogfile.log instead of rm /var/log/myhugeactivelogfile.log)

Sometimes you can even use this to recover an accidentally deleted file, if it's still held open in a process. You can go to /proc/$PID/fd, where $PID is the process ID of the process holding the file open, and find all the file descriptors it has in use, and then copy the lost content from there.