It basically consists of two fittings that have flat faces (surfaces) that are brought closer when one is screwed into the other. Between the two you put one or more rubber washers. You put the fitting over the tube and ss you tighten the fittings together they squeeze the washer/s and the whole inside them contracts forming a tight seal. Make sure you tighten it well. If done correctly there are no leaks whatsoever even at 50 bar.