Occlusion Training - Occlusion Training consists of deliberately restricting the blood flow to specific parts of the body while training them. This allows the body to reach fatigue much more quickly, because blood flow (and thus oxygen, and thus energy) is restricted. The body can continue to pump arterial blood into the area, but not allow venous blood to escape, allowing it to build up quickly. Once the muscle has been exercised, removing the restriction (a tourniquet, pressure cuff, knee wraps, etc.) allows the blood to escape. This method can potentially increase the rate of muscular hypertrophy.
Sometimes referred to as "Blood Flow Restriction" or BFR (acronym of the same), kaatsu training, occlusion cuff training, ischemia training, or more rarely, tourniquet training.
This is generally considered an advanced training technique because of both potential risks (you are deliberately cutting off blood flow, and must ensure you can restore it in a timely fashion) and limits on use (you can't do this except on limbs). If done, you must be careful to restrict bloodflow enough to block the bloodflow of the veins but not the arteries - it's a pressure restriction not a tourniquet to stop blood loss from a traumatic injury. Nonetheless, it seems to work in all populations. Its efficacy has been largely tested in a controlled fashion on untrained people.
This is one to be very careful with, as you are balancing potential gains vs. possible injury, if this article is accurate.