This teen gave Derek Law his rally rag. You know what happened next.

As Monday night’s playoff drama unfolded, 16-year-old Louis B. and his father David desJardins, from Burlingame, sat mere feet from their favorite team — one row behind the Giants’ dugout — but could hear only the boom of a hyped crowd behind them.

David told SFBay:

“It was so loud, honestly you can be yelling at the players and its hard for them to hear you.”

But even the loudest of cheers and screams couldn’t drown out Derek Law‘s enthusiasm. In the eighth inning, amid the Giants’ steep comeback against the Chicago Cubs, Louis mustered all the adrenaline he could to connect with the reliever:

“I yelled at Derek Law and said ‘Do you want my towel’? and he said ‘Sure!'”

David was shocked as he watched his 16-year-old son break a figurative wall typically enforced rigorously by ballpark security as Louis passed his orange rally rag down to the amped reliever:

“I said something like ‘What are you doing? should you be doing that?’ Throwing things onto the field makes you a bit nervous like that.”

Law is a Pittsburgh native, so the Terrible Towel ju-ju went into full affect. Law hoisted himself above his teammates in the dugout, waving Louis’s towel like a maniac while Conor Gillaspie and Brandon Crawford rallied to give the Giants a stunning eighth-inning lead. Gillaspie became the first lefty to triple off Aroldis Chapman.

Every dugout needs a cheerleader; Law said after the game he just wanted to rally the troops. And he did it with Louis’s help, the teen told SFBay:

“I’m not gonna say it changed anything, but I think maybe giving him the rally towel pumped up the dugout.”

Waving Louis’ rag with abandon, Law seemed to become just another one of the 43,571 enraptured fans, David added:

“It was just sort of a connection to the player. He was excited the same way the fans were so we felt like it was a common experience.”

Law’s unexpected role as an emotional spark started with his two innings of no-hit ball. The typically cool 26-year-old spun and punched his chest — a loud message, perhaps, to every writer and pundit that chided him and his bullpen brethren as the unlikely postseason set sail.

Through the Giants’ 10th straight elimination-game win, capped by Joe Panik‘s 13th inning walk-off double, the rally rag remained clutched in Law’s hand. Louis was too amped to notice as his favorite team streaked out of the dugout at midnight, but saw images in the aftermath:

“I was caught up in the moment, but I looked at pictures after and said ‘hey, there’s my rally rag.'”

Louis and David happened to bring a Sharpie to Monday night’s game. Law signed his name and ‘NLDS Game 3″ and tossed it back to the pair.

Law said after the game he wouldn’t mind getting the rag back, and the family was willing to hand it over. But Law insisted they keep the valuable prize. It had done its job.

And Law might have secured his new biggest fan, Louis said:

“My new favorite player might be Derek Law.”

Giants fans might be happy to know that Louis and David made it back to AT&T Park for Game 4, though they’re sitting in different seats.

They made sure to bring the signed rally rag, too.

Shayna Rubin is SFBay’s San Francisco Giants beat writer. Follow @SFBay and @ShaynaRubin on Twitter and at SFBay.ca for full coverage of Giants baseball.