How do you use it? What does it do? Do you pass finished wort through it hot or cold?
A hopback is a sealed chamber into which you put whole leaf hops. Hot wort exits the kettle, passing through the hopback before chilling.
Like whirlpool additions, the hops contribute volatile aroma compounds that would normally evaporate in the boil. The leaf hops also filter hot break, helping to clear your beer.
Any sealed chamber large enough to hold a few ounces of whole leaf will do. Probably 10-20 grams per gallon of wort. Add an inlet and outlet and put some sort of screen on the outlet. The one pictured above is good for gravity flows, but might not work as well for force pumped wort.