Ok, here it goes:
Caster and target level equal: Nothing.
Caster has 1 more Astral than target:50 points of stun damage to target
Caster has 2 more Astral than target:100 points of stun damage
Caster has 3 more Astral than target
eath
Even stun damage can be pretty bad, mages getting knocked out prematurely can be a bad. Besides, if the target has already fatigue, he migth take damage, possibly end up dead.