Compression fittings work by the compression of an ‘olive’ between two tapered surfaces and the pipe itself. The two surfaces are the body of the fitting (be it a valve, connector or any other type) and the nut. Using a spanner and a pair of grips the nut is tightened. This exerts pressure on the olive and bites it onto the pipe. Compression fittings work well if the pipe is clean and cut properly.