With the beginning of summer, pools all over the US are opening for recreational swimming — but in the Navy, recruits are getting ready for the brutal Basic Underwater Demolition/SEAL training, or BUD/S, that will turn some of them into Navy SEALs.

In the SEALs, where recruits of the elite special operations unit are pushed to their limits, there is no room for inefficiency. So it developed a more efficient swimming stroke: the combat swimmer stroke.

The stroke combines the best elements of breaststroke and freestyle to streamline a motion that not only reduces resistance on a swimmer's body, but makes the swimmer harder to spot underwater.

Here's a sample of the stroke:

Unlike freestyle, the combat sidestroke calls for the swimmer to stay submerged for most of it.

To do the combat swimmer stroke, dive in or kick off as you would in freestyle, but at the end of your glide, do a large, horizontal scissor kick instead.

Now comes the unique part — as the horizontal scissor kick tilts your body so that one arm is slightly higher than the other, pull that arm back while leaving the other outstretched.

Turn your face up toward the surface as you pull that arm down, take a breath, and begin to pull down your other arm. Another scissor kick, then reset your arms. You should not switch your orientation or the order in which you pull back your arms.

Here's a step-by-step breakdown:

Using the combat swimmer's stroke, Navy SEALs can go for miles in grueling training events that push their physical and mental strength.