Greenskins are creatures of little brain and almost no curiosity. Their single-minded enthusiasm for violence makes it very hard for sorcery to get a grip on their consciousness. Although they do have magic it is very different to the magic of humans and Elves. Their power comes not from the treacherous winds of magic, but from the inner psyche of the greenskin racial mind.
Obviously no Orc is going to bandy about a weak-kneed Elfy phrase like 'racial mind' or 'inner psyche'. As far as greenies are concerned, the power of their shamans is god-given. The god (or rather gods) in question are Gork and Mork - the eternally squabbling brother-gods of Orc and Goblin legend. Greenskin Shamans are links between their tribe and its quarrelsome gods. By working themselves into a deep trance Orc and Goblin Shamans can communicate with the great gods and orcish spirit creatures. Sometimes they meet ancient predecessors in the same way or they encounter other Shamans who happen to be wandering in the spirit-realm at the same time. The Orcs and Goblins call this spirit-realm the 'Great Green'.
Shamans
In the Warhammer game Shamans cast spells just like other kinds of Wizard. They have their own type of magic called Waaagh! Magic.
Shamans have spells (1, 2, 3 or 4 in number depending on the Shaman's Level) that are randomly selected from the list of Waaagh! Magic spells given later. Each Waaagh! spell is more or less difficult to cast as indicated by the usual casting number. All this is the same as other Wizards and doesn't present any new difficulties.
Where Orc and Goblin Shamans differ from ordinary Wizards is that their magical power comes from the mental energy generated by the greenskins round about. Every Shaman can access energy through the Great Green, but localised energy makes a difference too. If the Shaman is close to lots of greenskins, his power is increased as he picks up Orcish vibes from his comrades. This is normally a good thing - though it can be a bad thing if those vibes come from fleeing Orcs whose minds are panicked and confused.
For each Orc unit (of any type) of 10 models or more, or each Goblin unit (of any type) of 20 or more models, which is within 12" of a Shaman and engaged in close combat, the player adds +1 Power dice to his pile in his own turn, and + 1 Dispel dice in the enemy's turn. Note that you should use the number of models not the Unit Strength, and that if there are greenskins on both sides then all of them count for extra dice for both sides.
If a Shaman rolls a Miscast then he must roll on the special Waaagh! Miscast table. This is where the proximity of fleeing greenskins becomes a problem. Instead of rolling just once and consulting the table (as you would for ordinary human Wizards) roll once and once more for each fleeing unit of 10 or more Orc models, or 20 or more Goblin models within 12" of the Shaman. Again, greenskins on both sides count. Take the lowest result and apply it - ignore the other results.
Night Goblin Shamans
Night Goblins are experts on different types of fungus, so it's hardly surprising they know of the rare Shaman Mushroom which helps them tap the Great Green's power. In battle, each Night Goblin Shaman will have one piece of Shaman Mushroom. A Night Goblin Great Shaman will have D3 pieces. Each piece may be eaten (only once, obviously) to add D6 to the casting roll of a single spell. Decide after you have rolled your normal Casting dice and add the D6 result to the total. This is the final Casting roll. Your Shaman may continue to eat pieces of Mushroom and add dice until he has none left. Note that 'Mushroom dice' don't count towards the limit of Power dice used per spell. The extra dice may cause the spell to be cast with Irresistible Force. However, if any 'Mushroom dice' is a 1 then the spell is Miscast.