There's ways to recognize which spell or maneuver is being used: is this needed before using the Shadow Pin counter?
Or do you just instantly stop it while it's happening?
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You instantly stop it while it’s happening. The wording in the maneuver requires that it be initiated when the target is “leaving its space” but its effect is only functional if it is resolved before the target has actually left. Barring some kind of effect that hides the fact that the target is leaving a square (e.g. the 3.5e warlock’s flee the scene invocation, which left a 1-round illusion of the warlock in the warlock’s space), you should be able to use shadow pin any time you have it readied, not expended, and someone triggers it.