Devaluation is a policy act, not a market move, which is what distinguishes it from depreciation. It happens when maintaining a currency-peg has become too expensive: reserves are draining, domestic rates have been pushed to levels the economy cannot bear, or the real rate has become uncompetitive.
The intended benefit is a cheaper export price and a repaired trade balance. The costs are immediate: imported inflation, and a jump in the local-currency value of any foreign-currency debt, which is the mechanism that turned several currency crises into banking crises.
Devaluations are usually denied until the moment they happen, for the obvious reason that an admitted one would be front-run. This is why official denials carry little information.
Example: a peg is moved from 6.00 to 7.20 per dollar, a 20% devaluation of the local unit. A company with $50m of dollar debt sees its local-currency obligation rise from 300m to 360m overnight while its revenue is unchanged.
Related: revaluation, currency-peg, depreciation, capital-controls