These tensioning devices are to fit bikes with a derailleur hanger. They are held in place with a bolt which screws into your mech hanger, then they are fixed to the dropout with either your quick release skewer (there's an additional end cap supplied for this purpose) or hub axle bolt.

Their main advantage is in their simplicity, where setup takes literally a few seconds, and another great benefit is that they offer additional support for the derailleur hanger making it almost impossible to break.