A domino field, or D-field as it is commonly known, creates an even more complex distortion pattern than a normal Harelquin [sic?] holo-suit, shattering the wearer's image into a cloud of blindingly bright shards of multi-coloured light. A model wearing a D-field gets a 4+ cover save, just like a holo-suit. In addition, any enemy attacking the model in close combat always needs a roll of a six to hit, regardless of the models' comparative weapon skills. If the enemy has a fixed to hit roll which is not affected by Weapon Skill (Khârn the Betrayer, for example) then roll a D6. On a roll of 1-3 they are affected by the d-field and need sixes, on a 4-6 they are unaffected and use their normal to hit roll.
D-field
Source: Codex: Harlequins, p. 11