WASHINGTON (CBS DC) — Recent photos have caught President Barack Obama exchanging challenge coins with military personnel during a “secret handshake” that is a revered moment for photojournalists to capture.

The “secret handshake” exchange of the “challenge coins” is a longstanding military tradition in which the U.S. president tucks one of the small medallions into the palm of his hand to give to a stone-faced Marine or military member.

Two weeks ago, photojournalists managed to capture the secret exchange between Obama and two military servicemen in consecutive days, AOL.com reports. Just moments after the president landed at the Little Rock Air Force Base in Arkansas, longtime Associated Press photographer Susan Walsh snapped a photo of Obama handing a challenge coin to Col. Patrick Rhatigan, a commander of the 19th Airlift Wing.

“This was the first time I successfully made a photo of the president exchanging a coin,’ Walsh, a photojournalist with the AP for 23 years, told AOL.com. Walsh was a member of the AP team that earned a Pulitzer for its coverage of the Clinton-Lewinsky scandal.

Walsh admitted that she knew the coin shake “was going to happen,” but catching the actual exchange via photograph is always a challenge.

“You never know if you’re going to get it. It is kind of like photographing somebody winking,” Walsh told AOL. “For me, making a picture of the coin exchange is just like photographing a magician when you can see how they do their trick. It is just very rare and cool to know that you made that photo.”

Continuing his trip west, mainly for fundraising efforts, Obama was photographed making another exchange at the Air Station Miramar in San Diego just one day later.

AFP/Getty photographer Brendan Smialowski captured the exchange with Sgt. Maj. Richard Charron, a U.S. Marine.

“It felt good to meet the commander in chief,” Charron told AOL.com in an email. He noted that the challenge coin gift from Obama came as a complete surprise, but meeting a president was nothing new for him.

“Receiving the coin was a benny. During my career I’ve actually had the privilege of meeting four other presidents — George H.W. Bush, George W. Bush, Bill Clinton and Ronald Reagan.” And though this was Charron’s first challenge coin from a president, he has collected nearly 250 coins throughout the course of his lengthy military career.

In 2012, a challenge coin exchange was not such a “secret” handshake. Obama attempted a hand-off to Sgt. Kristie Ness just outside the Marine One helicopter, but instead the handoff was botched and Obama was caught by two photographers laughing about the faulty exchange.