The HOWTO has all the details. I've only done this
as a test example and have not used it extensively.

The pppd processes have to be
attached to a pseudo-terminal; std I/O won't work.
So the pppd's are opening say /dev/ttyp7 instead
of /dev/modem. This fools them into thinking it is
a real device. If the remote or local machine is not
Linux, there will be some variation of the commands.

Note, the ssh processes can also have -L and -R
redirs acting as "sidebands" in this setup. Perhaps
the sideband is used in emergencies, or for performance