In constrictive pericarditis the pericardium is rigid meaning it can't move inward or outward.
In early diastole, the pressure inside the pericardium is lowest leading to rapid ventricular ﬁlling resulting in a sharp y descent.
Now contrast that with cardiac tamponade in which active pressure is being exerted on the heart throughout the cardiac cycle.
The pericardial pressure exceeds that of the diastolic pressure leading to decreased diastolic ﬁlling and there is attenuated y descent.
To understand the concept, imagine that there is an empty glass bottle with a balloon inside it threaded at the opening. If you create negative pressure inside the bottle & try to fill it with water then it will fill rapidly.
Now fill the bottle with water and then thread the balloon onto it. Ii will be difficult to fill the balloon since the water in the bottle is exerting pressure onto the balloon.
Hope this helps.