AGT Standouts Sebastián and Sonia Talk ‘Crazy’ Experience of Receiving Simon Cowell’s Golden Buzzer

Simon Cowell bestowed his golden buzzer on an act that defied injury to capture the hearts of all the America's Got Talent judges.

Cowell, 64, awarded the honor to aerial act Sebastián and Sonia on the Sept. 3 episode of the hit reality competition series following an injury that forced Sebastián to perform a solo routine.

"It was crazy," the Barcelona-based contestants tell PEOPLE backstage following their golden buzzer moment. "It's been so hard, but now it's worth it."

Sonia fractured her elbow during a dress rehearsal, forcing her partner to take the stage on his own. His gravity-defying skills received standing ovations from judges Cowell, Heidi Klum, Sofia Vergara, and Howie Mandel.

"I was just breathing and telling myself, 'Okay, keep calm. I have to do this for us, for the group,' " Sebastián recalls about taking the stage alone, which Cowell referred to as "the definition of courage."

"I just tried to focus again on the act, and when I heard the audience, they gave me a lot of power because I was feeling weak and nervous," he admits, noting that the pair were both in tears before the performance began. "It's been long days, hard days, but as soon as the music started and the cheers of the crowd, I was feeling it. It gave me a lot of strength."

Sonia watched the performance backstage with AGT host Terry Crews, and reveals how "amazing" it was to see her partner in action. "It was hard for me, but all my soul was on the stage with him," she shares. "I said, 'I'm staying with you' in feeling."

"She was with me all the time ... it was a duo performed by a solo," Sebastián adds, noting his exhaustion about completing the number. "I was feeling tired and I didn't know what was going to happen, but when [Cowell] pressed the button, we started to cry because now it's like everything was worth it."

"I wasn't expecting to do it," Cowell tells PEOPLE backstage about his decision. "But when I saw him after hearing the story, I was blown away. I thought, 'that's how you do it.' He was incredible."

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The other judges were equally as blown away by Sebastián's efforts, with Klum, 51, stating that he "did his partner proud." "It's a real triumph to just say, 'I'm still going to go in this competition,' and hopefully something magical is going to happen. It did."

"What they do as a couple is incredibly dangerous, is incredibly beautiful, and the fact that the show must go on, and he made it just as elegant and beautiful as it could be," adds Mandel, 68. "There was something more emotionally wonderful about it."

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Sonia is determined to perform in the two-night finale, beginning Tuesday, Sept. 17 and concluding Tuesday, Sept 24, but she knows that there is a lot of work to be done before she is ready. "Every day I push in the gym," she explains about her difficult healing routine. "Rehabilitation and a lot of positivity."

"It's going to be harder for her," Sebastián adds about their upcoming live show return. "But she will get it. It's going to be hard, but it's going to be worth it."

America's Got Talent airs Tuesdays at 8 p.m. ET on NBC.