Pipe Inserts (or Pipe Stiffeners as they are sometimes known) are used to strengthen the side walls of plastic water pipe when it is connected to push fit fittings. They are not required when connecting pushfit fittings to copper pipe as the pipe walls are strong enough to maintain their own shape. An insert basically pushes in the end of the plastic water pipe and holds the pipe in perfect shape so the grip ring and seal within a pushfit fitting can securely seal the fitting without fear of the pipe deforming and potentially bursting off under pressure. The use of pipe inserts in MDPE water pipe is no different to that with white or grey water pipe. Several types of insert can be found here and brands are inter-changable