You've probably heard of most of the competitive swim strokes: freestyle, backstroke, breaststroke and dragonfly. Then there's the sidestroke, which is used in recreational swimming, and the doggie-paddle, just about every new swimmer's first lesson.

There's another stroke taught to Navy SEALs called the combat side stroke, which enables swimmers to swim for miles without tiring, even while weighed down with heavy packs. With the exception of breathing, the stroke is done entirely underwater. (No splashing means the swimmer is less likely to be seen by enemy eyes.)

To really improve training, try using this stroke while fully clothed. Then add boots. Then add a weighted pack. Navy SEALs have to swim 500 meters in under 10 minutes before they can move on to the Basic Underwater Demolitions/SEAL school. Can you keep up?

Here's a step-by-step breakdown of how to do the stroke:

Step 1: Glide. Arms are outstretched in front, feet together. Engage the core to keep hips level with head. (Hips lower than the head increases water resistance.)

Step 2: Catch water with one arm. The shoulder and bicep of this arm will turn towards the surface. Remember to keep both hands, hips and feet submerged at all times.

Step 3: Breathe and catch water with the other arm. Remember to be quick but gentle — you don't want the enemy to spot you!

Step 4: Scissor kick and reset. The top leg should always extend forward during the kick. If you're wearing flippers, you would be flutter kicking the entire time.

Kathryn Howard is Assistant Editor of AQUA Magazine.