Editor’s note: We filed this piece for PGATour.com’s Equipment Report. 

His 18-foot, curling left-to-righter breaks toward the hole, eyes locked on its path, Jon Rahm raises his Odyssey Rossie S putter and unleashes jubilant fist pump as his ball dives into the darkness.

We’ve seen the highlight how many times in the handful of days that have passed since that putt clinched Rahm’s U.S. Open victory?

It’s hard to imagine after seeing the confidence and firm conviction the ball would roll inevitably into the hole Rahm displayed on Torrey Pines’ 17th and 18th greens, Sunday, that the world No. 1 only switched into the flatstick the tournament prior to the U.S. Open. It’s surprising, too, that the mid-mallet model he settled on was a significant departure from the gigantic rear-center of gravity, high MOI mallet he had been using for months.

So, how did we get here? How did Rahmbo look more like 2008 Sunday Tiger Woods on the 72nd at Torrey Pines and less like a golfer who was so frustrated with his putting he went back to the drawing board less than a month ago?

The week prior to the Memorial Tournament presented by Workday, Rahm visited with Callaway head of tour operations Tim Reed and Odyssey rep Joe Toulon at the Ely Callaway Performance Center in Carlsbad, California, to test putters. There, Rahm was most intrigued by an Odyssey Rossie S mid-mallet putter. He remained happy with the Microhinge Star insert that had been, well, inserted into his 2-Ball Ten at the PGA Championship, so Toulon and company had the Rossie built with the Microhinge.

After evaluation on SAM PuttLab and Quintic (two putting analysis systems), it was clear the Rossie performed better than the higher-MOI, rear-CG 2-Ball Ten he had been putting with since joining Callaway’s tour staff in January. And as evidenced by his barnstorming three rounds at the Memorial Tournament and his clutch putt-filled win at the U.S. Open, the Spaniard’s putting performance was indeed elevated.

For the inside story of Rahm’s Rossie S, GolfWRX spoke with Odyssey tour rep Joe Toulon.

GolfWRX: When Rahm signed with Callaway, he was using a putter that looked very much like the Odyssey 2-Ball Ten he ultimately put in the bag. It intuitively made sense that’d be his choice, but he switched to a different putter at the Memorial. Why?

Joe Toulon: When he came into our putter studio in January, he hadn’t really been putting great. He was anxious to get into something. We had, probably, 20 putters made up for him, and the whole time, we were thinking the 2-Ball Ten with the S-neck would be the winner because it was similar to what he was using coming in.

But through that process, you have to listen to what the player is saying and how they’re saying it. He was struggling with setup and how his putter sat on the ground…and he found himself fidgeting.

In his college days, he used a 2-Ball. So the 2-Ball Ten, the way it sat on the ground for him was the reason he gravitated toward that. He felt comfortable with it…and with his path, he squared it up a little bit more and hit more putts in the center of the face.

The last thing we did with that putter was change to a White Hot insert. He’s such a feel player, and he told us that White Hot felt good at impact.

So that’s what he switched to at the Farmers Insurance Open and used through the PGA Championship.

Read the rest of the piece on PGATour.com.

*Featured image via Callaway’s Johnny Wunder