When a big mob of Orks charges they form a solid mass, blazing away with their shootas and screaming Waaagh! at the top of their lungs. The sheer momentum of their charge is stunning and opponents are in danger of getting bowled over by a scrum of Orks all trying to get into combat first.
To represent the Power of the Waaagh!, when an Ork mob charges into close combat check its size by rolling 2D6. If the score is equal to or under the number of Orks left in the mob they charge in with a rousing "Waaaaaaaagh!". All the Orks in the mob double their Initiative characteristic for the rest of the assault phase. In future assault phases the Orks revert to their normal Initiative values. If the 2D6 roll is greater than the number of Orks left in the mob the Orks charge in as normal and attack with their basic Initiative value.
Orks who make a sweeping advance into the enemy also use the Power of the Waaagh! The test for this is made at the beginning of the assault phase so any casualties from enemy fire will reduce the chances of them managing to maintain their momentum.
Errata — Chapter Approved: Codex: Orks Q&A (v1.0): Killer Kans do not get to test for the Power of the Waaagh!, despite being listed as a Mob.