I think it adds +4 to attack but I'm not sure of it's effects on defence. I think it has a positive effect on their morale. I'm all of it's effects are positive. It's that extra before they run into battle.

By the way, if the enemy is charging at you and you've finished your warcry, I recommend that you charge straight back at them. The warcry gives a brilliant charge bonus so it's almost definite your charge will be more powerful so you'll do loads more damage.

If it is cavalry that is charging towards you, remain still, this absorbs the charge better than if the unit is moving.

Hope I've helped.