well... two books and 5 websites later I'm quite confident about this... although the -chau form is kinda complicated but I'll try to explain anyway since it took me so long to understand it myself...
the ending derives from the word しまう shimau (finish) and the te-form of the verb... so it literally means "to finish [verb]"... but it would be way too easy, if the literal translation was the meaning (してしまう shite shimau "to finish doing something" is the exception)... first of all - this is, what I'm talking about:
to eat:
食べる
taberu食べて
tabete食べてしまう
tabete shimau食べちゃう
tabechauto know:
分かる
wakaru分かって
wakatte分かってしまう wakatte shimau
分かっちゃう wakacchau
so "-te shimau" becomes "-chau" (don't forget the っ! it doubles the following konsonant) in casual speech... it's like the difference of "I am" and "I'm"...
meaning: although there is no future form in Japanese the verb "to finish" implicates connection to the future... so 食べちゃう tabechau can be translated as "I will eat" and adds emphasis to the verb (literally "I finish eating")... it is also used to show regret, shock, sudden emotions or for warnings and suggestions... the -chau form seems to have a rather negative connotation... here some examples:
バスが行っちゃう! basu ga icchau! - there goes the bus! (literally "the bus finishes going")
鍵を忘れちゃった! kagi wo wasurechatta! - I forgot the keys! (literally "I have finished forgetting the keys)
now 分かっちゃう wakacchau makes sense but there's still the mystery of 分かっちゃない wakacchanai... it's of course the negative of 分かっちゃう but the correct grammar would be 分かっちゃわない wakacchawanai... aaanyway ^^
btw... it sounds kinda cute and mischievous to use the -chau ending instead of -te shimau... young girls (and interestingly - older men) usually use it but it's ok for men to use it, too... they just won't say it