“This procedure can be likened to a hike with no navigational aids
along a narrow, tortuous trail through a forest obscured by fog so
impenetrable that one can see only one step ahead. As long as one
remains on and follows the trail exactly,
there is no ambiguity concerning the proper direction to walk in. As
soon as one leaves the main trail, however, then returning to the
original trail would be impossible barring incredible luck. One way to
leave the main trail “reversibly,” that is, so
that one can find one’s way back to it, would be to carry some marker,
say, a length of string, one end of which would be staked to the main
trail and which would be unwound as one walked into the forest. One
would then simply follow the string back in
order to return to the main trail.
“
This paragraph comes from the patent (link above). Obviously, I am being slightly facetious and the patent itself provides all the gory details.