Setup(フリ、基本的には奇数の小節)とPunchline(パンチライン、偶数小節)を繰り返すと言う話らしい
https://www.quora.com/Is-a-punchline-in-rap-a-specific-phrase-that-stands-out-or-two-phrases-related-to-each-other
https://www.reddit.com/r/makinghiphop/comments/2ifjjr/how_do_you_guys_come_up_with_punchlines/