"His philosophy is very personal," Rahm said. "He likes to work with emotion and see what emotional state you are better off playing in.
"We have been working recently on new things that I really don't know how to explain.
"He gets into quantum physics, which is kind of complicated. It's a different way of thinking to traditional psychologists.
"It is basically different dimensions, with the first one being the physical dimension and the fourth one being the mental dimension. I have it written down and it makes sense when he explains it to you.
"That's why he is so good."
Well, this is how Del Carmen explains it: "Quantum coaching is based on the fact that we are all part of the whole, that our energy is creative and that, therefore, what is important is not what you do, but what you do and where you do it."
Rahm is a Del Carmen disciple. He concedes that, if anything will stop him reaching his potential - which can only be world No 1, considering the fact he has risen to seventh in the rankings in a little over a year as a professional - then it is his fiery nature.
At last month's US Open in Wisconsin, Rahm suffered a meltdown that has already gone down in the game's folklore. On his way to a missed cut, he pounded his wedge into the ground, before drop-kicking it, flinging his ball into the distance and then, on his way to the next tee, punching a sign three times.
The day before, he had hurled a bunker rake and launched a seven-iron into the air, as if it were a javelin. In the week afterwards, he was appropriately contrite.
Rahm, 22, rang playing partners Rickie Fowler and Hideki Matsuyama to apologise, and promised he would atone.
Del Carmen was consulted and the fix was made.
Alas, as ever, it was only temporary. At the Irish Open two weeks ago, despite being comfortably clear on his way to a six-shot win at Portstewart, Rahm boiled over in the final holes, slamming his putter into the turf.
At the very least, it showed he was consistent - as furious in glory as in mediocrity.
No, the perfectionist in him plainly cannot tolerate any type of failure.
"Joseba likes me to feel the emotion, but not to exteriorise it in that manner," Rahm said.
"What happened at Erin Hills was so weak and it was bad for me. Anger and letting my emotions not get out of control is something I've been working on since I was a kid.
"I'm really competitive and I don't like hitting bad shots. None of us do, of course, but it has taken its toll on me.
"When I feel everything is going well, I let my guard down and things like that happen. It really shouldn't.
"It's something I need to keep working on, and hopefully I can keep it up and amend it."
Rahm's rage almost led to him being kicked out of college. Unable to speak a word of English, the then-17-year-old from the small Basque town of Barrika was signed up by Arizona State University on a scholarship, but very soon head coach Tim Mickelson, brother of Phil, thought he had made a mistake.
In his very first competitive round, Rahm snapped, taking his fury out on his bag, breaking the kickstand as he kicked out. Mickelson, who is now Rahm's manager, forced him to run up and down the 60 steps in the college's American football stadium as punishment.
Rahm was told that if he was to continue his education, he had to do two things - learn the language and lose the attitude. While he has excelled at the former - his fluency is nothing short of incredible - he still struggles with the latter, although it is a mark of how volatile he was that those who know him claim he has made huge advances, since linking up with Del Carmen.
It is fair to say the pair have been good for each other.
The Spanish federation, fearing that their prodigy was on an inexorable path to self-destruction, initially fixed him up with established sports psychologists, but none could make any meaningful headway.
The answer was waiting in Rahm's own backyard.
"I grew up at the same golf course where Joseba played," Rahm said. "He has known me since I was young and I knew he was in charge of deactivating bombs, which sounded pretty cool.
"Once he retired, he discovered coaching and asked me if I wanted help, and he tried a couple of things and I said yes. I loved it from the very start - that was in 2014."
Rahm is, of course, something of a celebrity in the Basque community and has been since he was the world's best amateur. Del Carmen was summoned to assist Baskonia, the Vitoria-Gasteiz basketball club, who are run in tandem with Alaves.
"Mauricio welcomed me with open arms. He believes in this, because he had already worked with a coach on one of the teams he coached in Argentina, and he is a leader," Del Carmen said.
The Alaves players were not so sure.
"That is normal," Del Carmen said.
"They asked, 'What is this guy doing here?' Until they understand that you are not someone from outside the group, but from inside the group."
Pellegrino guided Alaves to the final of the Spanish Cup and boasted a victory over Barcelona in La Liga.
Such form persuaded Southampton he was their man and Rahm is convinced that Del Carmen could also have a positive effect on the south coast.
"He is doing well and people are realising what he has got," Rahm said. "If Pellegrino takes him to Southampton, he will do well there as well."
However, the Saints can wait. First, there is a sinner at Birkdale who needs his help.
