It’s no secret that Navy SEALs go through incredibly intense training.

The Navy’s Basic Underwater Demolition Course requires recruits to run and swim grueling distances, and the course’s Hell Week is famous for delivering 132 hours of unbelievable pain.

During their training, recruits can ring a bell three times to signify that they want to give up and be reassigned. Most of them choose to do so; only about 20 percent of recruits finish the entire program.

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For the recruits that make it through, the life of a Navy SEAL is difficult, to say the least. These sailors push their bodies to the absolute limits, and that’s a point of pride; they’re often considered to be the most physically fit soldiers in the United States military.

The Navy SEAL program first started in 1961, when President John F. Kennedy increased support for paramilitary operation funding. The SEALs focus on water operations, although that isn’t their only purpose. When deployed, they may have to swim long distances—and even if they’re not deployed in such a situation, they’re gluttons for punishment, and SEALs often look for ways to push their capabilities.

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The Navy SEAL physical fitness test only requires a 500-yard swim, completed within 12 minutes and 30 seconds. However, many SEALs need to swim for miles.

So, how do SEALs swim these incredibly long distances without tiring out?

As you might have guessed, the trick is a special swimming stroke, which combines elements of the freestyle and breaststroke. It’s designed to keep the swimmer mostly underwater, and every Navy SEAL knows just how it works.

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When the swimmer pushes off, he keeps his body tight, limiting resistance in the water. The swimmer’s hands stay very close together at the start of each stroke, then pull down, similar to the way that oars propel a boat. That downward pull is key, as it reduces muscle fatigue while allowing the SEAL to glide almost effortlessly underwater. The hands never go out to the side, instead staying low and pushing the water under the SEAL.

The swimmer breathes when pushing one arm down, conserving movement in preparation for the recovery. The “pull, breathe, kick, glide” process allows for fairly fast swimming but doesn’t drain the SEAL’s energy.

It also has another important feature: the limited movement allows the SEAL to remain undetected when swimming underwater. This has important implications in a combat setting; if a SEAL is in a position in which he has to swim to shore, he certainly can’t give up his position.

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It’s a fascinating process, and it shows how every aspect of a Navy SEAL’s training is tightly controlled. There’s not really much room for wasted movement, especially when a SEAL is still training—during Hell Week, you need every bit of energy that your body can muster.

Of course, as with any advanced physical technique, it helps to actually see the move in action. Here’s a quick video explaining the combat swimmer stroke; if you’re a swimmer, this is a great addition to your routine.