'Bobbing is a technique used to stay alive in water when the hands and feet are bound. Even if one doesn't anticipate ever being in a situation like that, this is a great technique to learn and use to build confidence and proficiency in the water. The principle behind bobbing is to expel all the air in the lungs during descent, and upon reaching the bottom, pushing off hard to make the ascent to the surface so you can quickly take a gasp of air, once again expelling it on the way down. Some people also like to expel just enough air so they can make the descent, then expel the rest on the way up. There's no set time as to how long you choose to stay at the bottom. You also want to stay relaxed, and not bend the legs, which would cause drag. Bobbing is most often initiated by jumping into the water to get momentum started. Many people panic during this exercise, which is why you should build up to it (i.e. don't bind people with rope).
The dolphin kick is another technique used while the hands and feet are bound, which allows you to maneuver through the water.