(covers information from several alternate timelines)
A multiphasic scan, or multiphasic sweep, was a method whereby a vessel's sensors could be reconfigured to penetrate areas of intense interference where conventional scanning methods would prove impractical.
In 2263 of the alternate reality, Montgomery Scott needed to reconfigure the USS Franklin's VHF output into a multiphasic sweep to cause a destructive chain reaction among Krall's swarm ships by confusing their unified cyberpathic coordination. Pavel Chekov reported that it would have to be on a frequency the swarm would not anticipate – "something loud and distracting," namely the Beastie Boys song "Sabotage". (Star Trek Beyond)
In 2371, Kathryn Janeway, captain of the USS Voyager, attempted to reconfigure ship's sensors to a multiphasic bandwidth in an effort to penetrate the strong electromagnetic radiation being emitted by a dark matter nebula in the Delta Quadrant, while attempting to track a vessel which had attacked one of their shuttlecraft. Tuvok later also used a multiphasic scan, especially the alpha k band to track the ion trail. (VOY: "Cathexis")
The next year, Captain Janeway ordered a multiphasic scan performed on a group of attacking ships controlled by the Bothans, in an attempt to discern any defensive weaknesses. The scans, however, revealed no useful information which the Voyager crew could use to defend itself. (VOY: "Persistence of Vision")
The same year, Lieutenants Tom Paris and B'Elanna Torres ran a multiphasic sweep in an attempt to locate the Cardassian ATR-4107 which the Voyager crew had discovered in the Delta Quadrant, after the missile masked its ion trail with a randomized EM field. (VOY: "Dreadnought")
In 2373, upon encountering a derelict Borg cube in the Nekrit Expanse, Janeway ordered Ensign Harry Kim to run a multiphasic scan on the vessel. Kim's scans showed no active energy signatures, and no life signs. (VOY: "Unity")