It is a weighted geometric mean of the dollar's value relative to other currencies:
Euro (EUR), 57.6% weight
Japanese yen (JPY) 13.6% weight
Pound sterling (GBP), 11.9% weight
Canadian dollar (CAD), 9.1% weight
Swedish krona (SEK), 4.2% weight
Swiss franc (CHF) 3.6% weight

So if the Euro crashes, by default, the USD will rise. Maybe the Jim WIllie theory of having the USD rise, rise & rise, the collapses could be true.