Essentially, Simon's noticed that there are lots of people that use year(someDate), but that the system doesn't understand that this function doesn't affect the order of the items in the index. month(someDate) does, but if you're already using year(someDate), then the combination of the two doesn't change.
However, the term 'invertability' is interesting, and ties into the Inverse Predicates concept that I've also used before, like at http://bit.ly/inversepredicates. The idea is that you might have applied a function to a column, creating a predicate that isn't sargable, but if you (or the system) can tell how to invert it, then you can make a predicate that can be easily handled by the Query Optimizer. Currently, the system doesn't understand the invertability of all the functions (even easy ones like the YEAR function), but it's something which would make SQL a lot faster if it did.
I'm going to let you read those various posts yourself, and encourage you to vote for Simon's connect item. But as well as that, I'm going to encourage you to consider the SARGability of the predicates in your query.
Edit: Simon's written a post on this now.