So, how did this happen?

How did the Giants upset the Patriots, 21-17, to win the Super Bowl?

Two words. Two simple words.

Tom Coughlin.

The man once criticized as being taciturn, aloof, regimented, and unemotional has proven himself to be none of those things. But he has proven himself to be an outstanding coach, one who inspires his players to believe.

Saturday night, Coughlin reminded his players that no one believed in them when they were 7-7 — except for him. He reminded them that all their efforts had been noticed, that none had been in vain. And he reminded them to finish strong, with pride.

“Talk about a room that was electric!” offensive tackle David Diehl told Ashley Fox of espn.com. “I mean, after that, you wish you could’ve played last night. I think we were all emotional sitting there listening to it. This is what legends are made of.”

Legend? Tom Coughlin? The man virtually fired by the New York media two months ago?

Yes. The same man who now has coached as many Super Bowl winners as Vince Lombardi and Bill Parcells.

You see, Tom Coughlin won’t be outworked or outcoached. The man who wears the tortured expression on the Giants sideline during games actually loves what he’s doing. And what could be more fun than rallying his team to one of the most amazing and unexpected runs in NFL history. Or, certainly, since his Giants did the same thing four years ago.

“I told the players, if you believe, good things will happen,” Coughlin said Sunday night. Maybe they believed because they had done it before. Maybe it was because the defense finally got healthy and started turning teams around.

Or maybe, just maybe, they believed because nobody works harder than their own coach.

Coughlin served as the Giants receivers coach under Bill Parcells from 1988 through 1990. As Parcells told Carlo DeVito in DeVito’s book, “Parcells: A Biography,” “He is one of my favorite guys I ever coached with. It’s simple, really. I like him. Of all the things that are very important in a coach, he is all of those things. And he is not afraid. That’s the main thing. He’s got conviction.”

Sports writer Mike Freeman offered DeVito the following anecdote: “On Christmas Day, Parcells called the Giants Stadium office to leave a message on Coughlin’s voice mail. The phone rang. No one would be working on Christmas Day. The phone rang again. Coughlin answered. He was the only coach at the office that day—had been for hours.”

“I don’t know if I should be proud of that story or not!” Coughlin told DeVito. “When we were together, I just gave him everything I had, which was in return for what he gave me. I’ve always tried to instill in my staff that your reputation is based upon your work ethic, and you have to prove yourself every day.”

Parcells taught Coughlin another lesson. Periodically throughout their careers, Parcells would send Coughlin a pair of gray socks.

“That’s it. Just a pair of gray socks in the mail,” reported Dave Hyde of Sports Illustrated. “When Coughlin played football at Syracuse in the ‘60s, the field was so muddy, white socks couldn’t be cleaned properly, and cost too much to replace. So players wore gray socks.”

“I smile when I get them,” Coughlin told DeVito. “What he’s saying is, ‘Don’t forget your roots, who you are.’”

And make no mistake: Tom Coughlin is the reason Eli Manning has become one of the elite quarterbacks in the game. It’s due in no small part to Coughlin’s unwavering belief in Manning.

Both arrived in East Rutherford for the 2004 season. Coughlin benched Kurt Warner midway through that season, and Manning has been his quarterback ever since.

When the going got tough in 2006, and the Giants’ best offensive player, Tiki Barber, chose to criticize both Coughlin and Manning, it was Barber who was soon gone.

One year later, the Giants were Super Bowl champions. They did it Coughlin’s way: a strong, ball-control running game and a stout defense, with a young quarterback charged with not making mistakes. Yet it was that young quarterback who coolly drove them to the winning score in the final minute against the undefeated Patriots.

Just like he did Sunday night. Manning wound up with better stats than his more heralded counterpart, Tom Brady. And it was Manning who made the throw of the game, a perfectly directed pass to Mario Manningham for 38 yards to start the winning drive. Of course, Manningham’s catch rivaled any in Super Bowl history, Lynn Swann and David Tyree included. But the ball was there.

Hard to believe that this team was 7-7 two months ago. But the Giants kept believing, because their coach believed in them.

“It’s easy to believe when you see how passionate Coach is about it,” linebacker Mathias Kiwanuka told Fox. “He’s at it every single game. He eats and breathes football.”

Even on Christmas Day.                

And now he’s made believers out of all of us.