The mammalian dive reflex is a series of neurological and physiological changes that take place when fully submerged under water.
Essentially, the body begins to preserve itself in order to survive under the conditions.
The heart rate slows, oxygen is preserved and blood is directed away from inactive muscle groups.
Activating the mammalian dive reflex aids in conserving oxygen stores and redistributing blood to vital organs.
This is often practiced by free divers in order to better adapt to deep water conditions.
It is recommended to put your head under in an ice bath first in order to more effectively activate the mammalian dive reflex, rather than waiting until the end.