Learn English – the origin of “sucker” and “it sucks”

I think what is going on in the Urban Dictionary definition you found is a description of someone at a carnival who is purposefully using reverse psychology in an attempt to actually get more people inside. I'm getting this from the definition's example sentence (Carnival Barker- "Don't go in there man it's haunted.") For example, such a carnival barker may target a man and a woman on a date. By saying to the man, "don't go in there, it's too scary," he could be seen as indirectly questioning the guy's "manhood," thus convincing him to go in and prove himself.

This is an admittedly confusing definition of carnival barker. For future reference, Urban Dictionary is often useful for very current slang and fun to play around with (try looking up your town or city), but it's not the best place for good definitions. Check out these "definitions" of Urban Dictionary at Urban Dictionary to get my point. And even for definitions of slang, be sure to check the poster's other entries for a sense of their credibility and take note of how many up vs. down votes their entry has.

The idiom "If and when X, (consequence)" has the meaning "if X happens, then when X happens, (consequence)". In other words, the "(consequence)" will not happen unless X happens, and will not happen before X happens.

It is generally used when talking about not making unnecessary preparations for an event that may not occur:

We'll come up with a zombie attack mitigation plan if and when we're attacked.

Some people might complain that "if and when" is redundant, because the "if" carries with it a sense of "when", in that you don't know whether X is going to happen until it actually does. But it's sometimes possible to know to some degree of certainty that something is going to happen, before it actually occurs - and in fact this is often the situation where the phrase is used:

We'll open fire on them if and when they open fire on us.

Simply saying "if they open fire on us" can invite the response "but we know they're going to, so we should fire now"; the when emphasises the speakers view that no pre-emptive action should be taken.

As to the difference between "if and when" and "when and if", the basic meaning is identical (as others have noted), but there is possibly a slight difference of nuance: the standard phrase is "if and when X", but inverting it to "when and if X" emphasises the if part, thereby putting more emphasis on the fact that it is uncertain whether X will actually occur.