Of course this is also Amir's opportunity to find redemption after not standing up for Hassan so many years before. The significance is that it shows that everything has come full circle and that Amir now has his chance to absolve himself of the guilt he has carried throughout his life. It is also significant because, unlike Baba, he has now had this opportunity to find a release, perhaps this is another reason he laughs and Assef finds it so infuriating.
This incredible release strikes Amir as funny because one might expect it from winning a fight or overcoming something. He has overcome his guilt but is getting kicked around at the same time and for some reason it strikes him as hilarious and coupled with that great release of guilt, he can't help but laugh.
Amir starts laughing while getting beaten up because this was his way of redeeming himself. All those years he felt so bad that he betrayed Hassan, he just wanted something back on him so he would feel better. When Amir throws the pomegrante at Hassan when they were younger, Amir wanted Hassan to throw it back at him, but he never did. Now he was serving for what he did and although his body was in so much pain, his soul had been healed.