When he can't tinker, he grows anxious. "If I don't do a set in two weeks, I feel it," he said. "I read an article a few years ago that said when you practice a sport a lot, you literally become a broadband: the nerve pathway in your brain contains a lot more information. As soon as you stop practicing, the pathway begins shrinking back down. Reading that changed my life. I used to wonder, Why am I doing these sets, getting on a stage? Don't I know how to do this already? The answer is no. You must keep doing it. The broadband starts to narrow the moment you stop."
...and why he thinks small bits are harder to make...
Seinfeld likens his fine-bore interest in jokes to his longstanding infatuation with Porsches, of which he owns “a few dozen.” “People ask me, Why Porsches? A lot of it is the size, same as with bits. The smaller something is, the harder it is to make, because there’s less room for error.” In high school he took shop classes, even after a counselor told him that collegebound kids didn’t need to, because he wanted to know how machines fit together. “I have this old ’57 Porsche Speedster, and the way the door closes, I’ll just sit there and listen to the sound of the latch going, cluh-CLICK-click,” Seinfeld said. “That door! I live for that door. Whatever the opposite of planned obsolescence is, that’s what I’m into.”
...and how there are different kinds of jokes in a set.
“There’s different kinds of laughs,” he explained. “It’s like a baseball lineup: this guy’s your power hitter, this guy gets on base, this guy works out walks. If everybody does their job, we’re gonna win.”
The baseball analogy reminded me of Louis CK discussing "brushback pitch" jokes.
It's worth checking this accompanying video too. In it, Seinfeld describes the anatomy of a Pop-Tart joke and shows his longhand writing process.