Antimatter spread
From Memory Alpha, the free Star Trek reference.
An antimatter spread is a harmless pyrotechnic display consisting of many small antimatter charges. Sensor interference from the deployment of an antimatter spread can serve as a way to confuse sensor readings and mask the launch of small craft.
In 2367, the separated saucer module of the USS Enterprise-D used an antimatter spread while it confronted the Borg following the Battle of Wolf 359. It covered the approach of a shuttlecraft containing Data and Worf, who transported aboard the cube to rescue Captain Picard, assimilated as Locutus of Borg. (TNG: "The Best of Both Worlds, Part II")
