Example
Inflation forecast 3.0%, actual 3.4%: a hot surprise.
In practice
Prices already reflect what the market expects, so the published forecast is the starting point for every release. A figure in line with it usually causes little movement; a figure far from it can move markets in seconds.
Traders check the consensus on the economic calendar before a release, so they can tell at a glance whether the actual number is a surprise.
What to watch for
The first reaction to a surprise often reverses within minutes as traders read the details and the revisions to earlier figures. Large traders may also be positioned for a number different from the published forecast.