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Futures markets show the order book. On the CME's euro futures, the best bid and offer sizes are visible, along with deeper levels. The ratio of buy-side to sell-side size is called imbalance, and some traders treat it as a short-term signal.

The idea: when the bid side is much larger than the offer, there's more support below, and price is more likely to tick up. When the offer is much larger, the reverse.

Research on equity and futures markets finds some predictive power over very short horizons, a few seconds. The effect is small, and fast machines already capture most of it. For a retail trader with a delay of several hundred milliseconds, most of the edge is gone before the order arrives.

Where it helps more is in judging conditions. A heavily one-sided book suggests a temporary lean, and a book that flips quickly indicates spoofing or uncertainty.

Spoofing, placing large orders with no intent to trade, is illegal but occurs. Large resting orders that vanish when price approaches should be treated with suspicion.

Imbalance also says nothing about the underlying spot market's liquidity, which is separate.

For those with access to futures data, imbalance is worth observing alongside the chart. Whether it can be traded profitably is a different matter.

Has book imbalance ever given a useful warning before a move on a spot pair?

I spent a few months watching euro futures depth next to my spot chart, mostly out of curiosity after reading the same research you mention. I never managed to trade the imbalance directly. By the time I saw a lopsided book and clicked, the spot price had usually already moved, and my fill was worse than if I had ignored it.

Where it did help was as a background read on the session. On days when the book kept flipping from heavy bids to heavy offers every few seconds, my spot scalps were noticeably worse, with more stop runs and fewer clean moves. A steady lean in one direction for several minutes was a better sign that the market had an opinion.

I also learned not to trust big resting orders a few ticks away. I saw plenty of them disappear just before price got there. The only size I came to respect was the kind that kept getting hit and stayed put, because that showed somebody was actually willing to trade there.