what i don't get about fizzbin

I'm down with Star Trek. And I'm down with fizzbin. Except on Tuesdays.

But there's one thing I don't understand about Captain Kirk's explanation. After dealing two jacks to the gangster he excitedly explains that that's a half-fizzbin already. The gangster guesses that he needs another jack. Kirk says no. That would then be a shralk, resulting in disqualification. OK so far.

But then Kirk deals him another jack and exclaims how lucky he is. That doesn't make sense.

If anyone knows the rules of fizzbin can they explain it to me?

Thanks.