He knew you were sat in the front, and therefore, he knew that you were watching him. He was somewhat distracted on stage, not enough to forget what he was doing.

Deciding he wanted to show off, he did a flip, earning a grin from you.

When he went back to the microphone, he couldn't help but notice that your friends were nudging you while you covered your face. He knew that you all discovered that that flip was for you.