Where Orc and Goblin Shamans differ from ordinary Wizards is that their magical power is focused by the mental energy generated by the greenskins around and about. Every Shaman can access energy through the Great Green, but localised energy makes a difference too. If the Orcs are fighting, a Shaman's powers are increased as he picks up the 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.
If one or more Orc unit of twenty or more models is in combat at the start of the Orcs & Goblins player's magic phase, the Orc player may add an extra dice to his power pool.
If one or more Orc units of twenty or more models is fleeing at the start of the opponent's magic phase, the Orc player loses a dice from his dispel pool.
In both cases only Orc units are counted; Goblin minds are too weak to make an appreciable difference.