A Dark Synchro Summon is a method of Summoning a Dark Synchro Monster. Both the Summon method and the monster category exist only in the anime and in the later members of the Tag Force series of video games. Its mechanics are somewhat the antithesis of a Synchro Summon, as instead of requiring a Tuner monster whose Level is added to the Level(s) of a non-Tuner monster(s) to equal the Synchro Monster's Level, it requires a Dark Tuner monster whose Level is subtracted from the Level of a non-Tuner monster to equal the Dark Synchro Monster's Negative Level. Therefore, in order to Summon a strong Dark Synchro Monster with a high Negative Level, the Dark Tuner must generally have a very high (positive) Level and the non-Tuner a very low Level, barring the activation of cards such as "Dark Wave".
In the anime, Dark Synchro Monsters have only been used by the Dark Signers and their possessed servants. When a character performs a Dark Synchro Summon, the Dark Tuner first transforms into its Level stars, which pierce the non-Tuner monster making it appear in pain. The non-Tuner monster then transforms into its own Level stars, which 'cancel out' some of the Dark Tuner's stars, disintegrating the non-Tuner monster in the process, which is shown to be excruciatingly painful for the non-Tuner monster.
The remaining Dark Tuner's stars then turn black and spin in a circle, before merging into a column of darkness that reveals the newly Summoned Dark Synchro Monster. Each character recites a Summon chant while performing the Summon; while this generally varies between characters in the original Japanese anime, all Dark Synchro Summon chants have been the same in the dubbed anime: "When the shadows are devoured by even darker shadows, the curtain pulls back and reveals a world without light!" which is Roman Goodwin's Summon Chant in the Japanese version.
In the Tag Force games, the first part of the Summon animation is skipped, and the dark stars appear from nowhere before spinning in a circle and merging as before.