Mets first baseman Pete Alonso singles against the Philadelphia Phillies...

Mets first baseman Pete Alonso singles against the Philadelphia Phillies at Citi Field on Monday. Credit: Kathleen Malone-Van Dyke

MIAMI — When J.D. Martinez signed with the Mets in March — and when he joined the team in April — he brought with him a long-standing reputation as a hitting savant, someone with an ability and willingness to observe and convey in a way that made teammates better. It was as much a part of his mystique as his actual hitting ability.

And now the Mets have tangible proof: It’s real.

Martinez helped Pete Alonso bust out of his deep slump recently with a friendly tip, a simple drill and “a hell of a mental cue” that provided “instant feedback,” Alonso said.

Upon returning to the lineup after their impromptu hitting lesson on May 6, that day in St. Louis when he was on the bench, Alonso has been hot: a .313 average with a 1.028 OPS and seven extra-base hits, seven RBIs and seven runs in eight games heading into the Mets’ weekend series with the Marlins.

“He understands the swing and can help pretty much everybody,” Alonso said. “He has a very unique perspective. Sometimes you need a different perspective to help.”

Martinez actually noticed Alonso doing something wrong at least a few days earlier, but he didn’t want to say anything — not yet, not so soon after joining the team following his personal spring training. He was just getting to know them, and them him, so he didn’t want to step on anybody’s toes.

But what he saw was critical: Alonso was “pulling off,” Martinez said, or over-rotating his body. He was turning too much toward the third base dugout — behind him — which made it difficult to produce the rocketed batted balls for which he is known.

“People call it direction. People can say he’s over-rotating,” hitting coach Jeremy Barnes said. “Basically, a lot of his energy and his body is going that way and he’s having to push his hands the opposite way. As opposed to everything working together to the big part of the field.”

Alonso said: “Over-rotating, yeah, it’s a problem. But it also causes a bunch of other things: not being on time, not seeing the ball, not recognizing [pitches and movement]. Being able to keep my direction to the big part of the field, that really cleans a lot of things up.”

When Alonso looked particularly lost — and was out of the lineup, a rare occurrence — and they were hanging around the batting cage, Martinez felt comfortable enough to speak up.

“Pete’s always been very open-minded to it, so I thought I’d take a shot and tell him what I saw,” Martinez said.

He offered an exercise to fix it, too. To prevent Alonso from turning too much, Martinez positioned him on the edge of the cage, his back just about touching the netting. When he swung or hit a ball off the tee, the goal was to not let his body reach that netting.

“Any time you feel your shoulder or back rotate into the net, then it’s bzzzz,” Alonso said, imitating a buzzer noise. “Bad. It’s good to have that instant feedback.”

Martinez said: “He loved the way it felt. ... All right, all right, don’t get too excited. Try it in a game and see if it works. Then he hit that ball and was in love with it.”

“That ball” came in Alonso’s only at-bat after coming off the bench on May 6. He hit a line drive at the second baseman, an inning-ending out that was so much more satisfying than usual.

If he just kept doing that, it would all work out.

“That didn’t mean much to a lot of people, but it meant a lot to me,” Alonso said. “It’s like, wow, I hit the ball hard to the big part of the field. I was trying to do that. And it happened.”

He has looked an awful lot like regular Pete Alonso ever since.

“It’s that challenge to stay in that mechanic, in that swing during the game and feel like, hey, this is an extension of BP,” Alonso said. “The game is an extension of BP. You’re going to make the same move . . . I’ve been able to buy in and trust in that process. It’s helped out a ton.”

Martinez said: “He listened, I guess. He understood what I was saying. He’s like, ‘No one has told me that.’ So try it.”

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