Strong bleeding can cause death in minutes. Bleeding from small wounds usually stops after a short time because the blood clots . But clotting cannot stop the flow of blood when a wound is big.
The best way to stop bleeding is to press on the wound itself. If possible let the person lie down and raise the bleeding part of the body. Then put a sterile handkerchief , cloth or towel on the wound and press it down with your hand. Do with for 10 to 20 minutes until help arrives.
Sometimes direct pressure cannot stop strong bleeding . If the leg or arm is hurt you can try to stop bleeding by putting pressure on the artery that carries blood to the injured body part.