Some mega armoured Orks get extra power boost units fitted to their suits to ensure that they don't miss out on any chances for a fight. If an Ork in mega armour has a mega boosta he is allowed to re-roll his dice for the distance he moves. If a unit of Orks in mega armour includes some with mega boostas and some without only the ones that have the boostas will gain the benefit of the re-roll, the others will only move the distance first rolled.
Errata — Chapter Approved: Codex: Orks Q&A (v1.0): The re-roll applies to any movement distance rolled on dice — this includes falling back, normal moves, assault moves, and Forced March, not just the standard Move phase.