It's only local. Think of a garden hose. If you squeeze the tip, water will squirt much further. If you squeeze it a meter up the hose, water will drizzle out. In the latter situation, pressure at the opening drops, but it increases before the pinching point. Vasoconstriction works the same way. There is less pressure on the opening as not to dislodge the platelet plug. But opposed to a garden hose the pressure before the constricted point dissipates through the rest of the system.