When the doctor takes blood from a vein in the forearm, he puts a tourniquet on the shoulder and suggests actively

When the doctor takes blood from a vein in the forearm, he puts a tourniquet on the shoulder and suggests actively squeezing and unclenching the hand. For what?

To take a blood test, the veins must be clearly marked. The active work of the hand causes blood flow to the forearm, but the tourniquet interferes with it, and its return outflow becomes impossible due to the venous valves, as a result of which the blood accumulates in the veins, they swell and become clearly visible.