The decreasing water pressure allows the lungs to re-expand. As the volume of the lungs increases, the pressure exerted by the oxygen remaining in the lungs drops. The oxygen can no longer enter the blood stream. When the blood oxygen levels become too low, an ascending diver may pass out before reaching the surface. Similarly, pockets of air trapped in the lungs can expand during ascent, possibly tearing the delicate tissue.
Air then invades the blood stream, resulting in immediate death.
Loic Leferme and his colleagues try to monitor each other's reactions closely for any signs they are nearing their dive limits. Free diving is clearly a dangerous sport.