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.