When you create a link to BusyBox for the function you wish to use, when BusyBox is called using that link it will behave as if the command itself has been invoked.
For example, entering
ln -s ./BusyBox ls ./ls |
will cause BusyBox to behave as 'ls' (if the 'ls' command has been compiled into BusyBox).
You can also invoke BusyBox by issuing the command as an argument on the command line. For example, entering
./BusyBox ls |
will also cause BusyBox to behave as 'ls'.