It negates only the first hit taken, no roll allowed , even if your Pluck Roll would safe you.
Example: shooting phase 1st guy hits you with a pistol, simply ignore the hit, even if your 3+ Pluck roll would probably safe you anyway.
later that phase second guy hits you with is gatling , now you must roll your Pluck safe with a -2 modifier.
Note: Only the first hit in the game is negated, melee or shooting.