The dollar index tracks the dollar against a basket of major currencies, heavily weighted toward the euro. That makes it a close mirror of EURUSD, but with some differences that can be informative.
When the index moves and a pair like GBPUSD or AUDUSD has not yet reacted, a short window might exist. Conversely, when one pair moves sharply without the index, the move may be specific to that currency.
Using it as a confirming signal is common. A long setup in EURUSD aligned with a weakening dollar index has more support than one fighting it.
The limits are practical. The index is built from futures or computed by a data provider, and its update rate may lag spot quotes. By the time it moves, spot might already have.
Testing is again the answer: record signals with and without index confirmation and compare results. If confirmation improves the hit rate enough to offset the missed trades, it earns a place.
Does a dollar index chart add anything to the decisions, or is it clutter?