Pressure change in atria
The curve you are talking about is JVPJuglar Venous Pressure Curve.
You are talking avoit x-decent after c pulse.
The downward deflections of x decent is produced by the atrium relaxation and the downward movement tricuspid valve.
This causes decrease in atrial pressure.
A new fascinating question could be why x-decent is greater than y-decent.
Good luck.