I looked at it and can't figure out how it's used. I think it's got to be some kind of lever, but damn if I can work out where you attach any wires or how it takes a strain...
They're handy for quick repairs. Easier to pack into the woods than a goldenrod or comealong, but those are able to pull a lot harder.
Where can you buy steel corner posts? Like the ones in callmefences photos.
Most use recycled drill stem pipe from the oilfields. Many metal suppliers carry it or can get you in contact with someone that does.
Can get it full length (usually around 30ft) or some places will cut it for you.