Activity in this zone is purely anaerobic, hardcore effort and can only be sustained for a short duration of perhaps 10-20 minutes. This exertion level takes you to the limit where your body begins to produce lactic acid. At this point, you've reached your lactate threshold, and your body switches to anerobic metabolism where fat is no longer used as the main energy source so the body switches to use stored glycogen instead.
However the effectiveness of anaerobic activity can be improved through training. In other words, it is possible to improve your intense endurance so that the onset of lactate theshold is delayed, which enables you to work longer at higher speed.
This is a very important zone for competitive athletes to use in their training regularly, as it allows your body to adapt to excess lactic acid in the bloodstream, enabling you to build speed, power, and strength. You will know when you've hit this zone because you will be unable to speak, except a single gasped word at a time. Sprint distance triathletes will race in this zone, and learn to keep time above lactate threshold for important tactical surges only. The ideal race pace lies just below the lactate threshold, where the body will utilise a mix of the aerobic and anerobic energy systems to maximise intense endurance.