See page 145 of Codex: Chaos Space Marines
Instead of following the normal rules for Rapid Fire weapons, models in this unit firing Rapid Fire bolt weapons make double the number of attacks if any of the following apply:
• The shooting model’s target is within half the weapon’s range.
• The shooting model is INFANTRY and its unit Remained Stationary in your previous Movement phase.
• The shooting model is a TERMINATOR or BIKER model.
For the purposes of this ability, a Rapid Fire bolt weapon is any bolt weapon (as defined on paeg 176) with the Rapid Fire type.