Harlequins fight almost as individuals, and rely less upon the coherency of their unit to defeat the enemy. A Harlequin model must stay within 4" of another model in the unit, rather 2" as is normally the case. See page 43 of the Warhammer 40,000 book for details of unit coherency.
Dispersed Formation
Source: Codex: Harlequins, p. 6