Drongos act as sentinels for other species, calling from a high perch when a raptor appears. The warnings are real often enough that meerkats and pied babblers respond to them, and foraging animals gain from having the drongo overhead. That established reliability is what makes the deception work.
When a target unearths a large prey item, a drongo may give an alarm call with no predator present. The forager abandons the item and runs for cover, and the drongo takes it. Crucially, individual drongos switch between their own alarm calls and mimicked alarm calls of the victim's species, which slows the rate at which targets learn to ignore any single call type. Field observations in the Kalahari found this tactic supplies a meaningful share of a drongo's daily intake.
The question that remains open is what the drongo understands about the deception. Flexible call switching after failed attempts is consistent with tracking the target's responses, but it does not by itself demonstrate that the bird represents another animal's beliefs.