As brother SIM pointed out, it's actually the method of jelqing and improper grips. Under erection, the veins and capillaries that supply your blood under natural constriction are under pressures. Those who just jelq blindly further pressure the veins and capillaries and cause thrombosis due to overpressurization without first preconditioning the veins and capillaries. It's like working 200lbs of weight as a newbie without first conditioning the body to deal with the weight loading that exceed the body potentials.