图说英语：跟老友记学英语口语1.1(28)

The "snap in his turtle" refers to sexual performance, it"s kind of a "clean" way of saying a vulgar idea (also known as a euphemism). A man who has no snap in his turtle is impotent; or, unable to have an erection.
I do have an idea on how it was created. "Snap," of course, has many meanings, but most of them have to do with making a quick, sudden movement (or sound), often of a closing or biting nature. In North America, there is an animal known as a "snapping turtle," which is a type of turtle that is known to bite (or "snap") humans and other animals. From what I have heard, if one of those things bites you, it hurts a lot. Now, if one of these turtles can"t snap, then there"s not so much to fear--if they can"t bite you, they can"t hurt you. A snapping turtle with no (abilty to) snap, then, is powerless, and from there the slang was created. The word "impotent," aside from its sexual definition, also means "without power." In the case of "Friends," they decided to use the word "turtle" in this way, and so a turtle without "snap" is understood by Franny and Monica to mean a man without (sexual) power or capability. /are you kidding? I take credit forPaul, you know before me, there was no snap in his turtle for two years. : if A takes credit for B,that means B isindebted to(负债的,感激的) A; here snap means vigor(n.精力,活力), while turtle is a vulgar expression to refer to male’s genitalia ,i.e. penis or peepee