A positive displacement pump can be understood as a pressure pump which causes the flow of liquid in a pipe or a casing due to some pressure being applied to the water particles to move towards a certain direction.
To make a better understanding of it, compare a positive displacement pump with a velocity pump. Centrifugal pumps are one example of the velocity pump.
In a centrifugal pump, the flow of liquid is generated not due to pressure on the water particles to move into a certain direction. Rather, the water particles are simply receiving a kinematic energy by turning of the impellers, even if to go into a rotational motion.
This rotational moment is converted into a translatory moment of liquid particles due to the shape of the volute casing. It is the shape of the casing, the Volute, which is the real magician in a centrifugal pump.
Thus, in a centrifugal pump, the pressure on the water particles to move towards a certain direction is very low. Therefore, in the casing in blocked, or the pipe line valve is closed somewhere down, the pipe is not going to burst away due to abrupt rise of pressure within the system.
However, in a positive displacement pump, the examples of which are the Reciprocating Pumps and the Screw Pumps, the water particles are pressed by the piston to move towards certain direction even if under pressure. The result is that, if the pipe is closed, the pressure will rise actually until the system ruptures.