Performance freediving is a dangerous leisure activity. The consequence of small misstakes can be fatal, both at competition, training and especially by those that has not yet taken a course in freediving. The main danger is to suffer lack of oxygen - which will cause loss of motor control and in extension also a blackout. Both these will probably occur at the surface. But if hyperventilating before a dive or extending your dive beyond your limits you might suffer shallow water blackout. Which could be explained in a decrease in waterpressure while descending the last meters which will cause oxygen leaving your blood and your brain and reentering the lungs - you blackout - lose your air - sink - and die.