There is an o ring in the fitting that is compressed onto the pipe making the seal and the fitting itself is pressed onto the pipe either side of the ring giving rigidity. The end of the fitting is also pressed round onto the pipe to stop any ingress of dust etc. All in one movement.
The joint is utterly solid, the tool fully automatic and even tells you if the press is not complete. 25 year minimum they reckon.
Just been getting a bit hacked off soldering under floors, flux etc this is quick and clean.