Waaagh! Once per game, the Ork player can declare a Waaagh! at the start of his Shooting phase. This may not normally be declared on the first turn as a proper Waaagh! needs some momentum behind it. For the duration of that turn, all friendly Ork infantry units have the 'fleet of foot' rule (not Gretchin units, they're far too weedy for a proper Waaagh!).
If a unit rolls a 1 when making this Waaagh! movement, the Orks start fighting before they get to the enemy. One model from that unit takes a single wound. Note that the unit may still move an inch, and assault as normal.