Let me build the scene for you: Jerry is walking down the street talking with Elaine (probably about nothing). The weather is a bit overcast but other than that it’s nice and not raining. 

A bit further down the street, he sees a man selling umbrellas on the sidewalk. The umbrella salesman has an umbrella out and open and is twirling it in his fingers. 

Jerry then brags that he was the person who invented “twirling the umbrella when selling it” when he worked as an umbrella salesman. 

He claimed that “the twirl” attracts customers. 

Notice earlier I mentioned that the weather was nice overall and not raining?

If it had been raining, then there probably would have been no need to do “the twirl”. 

Selling umbrellas is much easier when it’s raining. But when it’s not raining, you have to find a way to attract customers. 

Jerry invented “the twirl”. 

How are you selling your umbrellas when it’s not raining?

Bonus: Jerry mentions to the salesmen that he was twirling the umbrella too fast and it was “disorienting the customers”. 

So, not only has Jerry invented a way to get noticed, but he had (presumably) tested it also.