The way they work is by spacing the rear 02 sensor in the vehicles catalytic converter away from the excessive gas flow.
As their name suggests, they also contain a metallic core catalytic substrate. This acts as an additional catalyst for the exhaust system and can help with turning OFF the C.E.L (Check Engine Light)
Anyone can install them. Simply unscrew the rear 02 sensor from the catalytic converter that is causing the CEL code and simply screw in the mini-cat. Once the min-cat is nice and tight you can then re-fit the 02 sensor into the mini-cat. In essence they work as a double adaptor.
This particular mini-cat is set at a 45 degree angle for those tight spaces and has the EURO 4 specification.