A phaser sweep was a procedure in which a phaser emitter, typically mounted on the wall of a room, emitted a phaser burst that covered the entire area of the room, usually on a setting of three or four.
When this procedure took place, any Changeling present in the room would convert back to its gelatinous state, making the Changeling detectable.
This procedure was used again later that year when a Dominion infiltration of Changelings was thought to have occurred on Earth. These phaser sweep devices were implemented in all Starfleet and Federation installations on Earth under orders from Captain Sisko, who was temporarily promoted to head of Starfleet security. (DS9: "Homefront", "Paradise Lost")