"Just click the translated phrase and you'll see a pop-up menu of possible alternates for that phrase, as well as the original phrase highlighted in your original text. Not only can these alternative translations give you a better understanding of a confusing translation, but they also allow you to help Google choose the best alternative when we make a mistake," explains Google.
It's interesting to notice that the new feature helps you better understand the technology behind Google Translate. "Typically, when we produce a translation, our system searches through millions of possible translations, selecting the best -- that is, the most statistically likely -- translation. With this feature, we expose more of those possible alternatives."
Right now, Google only shows alternate translations when you try to translate a text, so this doesn't work for web pages.