A helpless defender is any creature that is attacked while being unable to defend itself, because it is asleep, charmed or paralysed. Many an assassin will approach a sleeping victim through stealth; this is often the easiest means of killing a dangerous opponent. Because these opponents are not rated "combatants," experience is not awarded for the damage done to helpless defenders.
When attacking a helpless defender, the attacker does not roll to hit. Instead, it is assumed the hit occurs, and that the damage done is 10-60% of the victim's maximum hit points above zero, each combat round. In the case of sleeping victims, the first hit will result in the victim awaking; but if the damage done is sufficient to stun on the first blow, there is little chance the victim will ever receive an opportunity to fight back, unless aided by an ally.
Assassins or thieves cause an addition bonus 10% per blow against helpless defenders, or 20-70% of the victim's hit points. Both assassins and thieves may attempt to assassinate or backstab the opponent; but if this fails, the damage that occurs is still a minimum of 20-70%.
Hiram, with a maximum of 20 hit points, has failed his save and is being held by the spell hold person. Iolanthe comes upon him, raises her sword and strikes; she rolls a 3 upon a d6, causing 30% of Hiram's hit points, or 6 damage. This is sufficient to stun Hiram, but it also breaks the spell. Miriam attacks again, only now she must roll to hit, because Hiram is no longer rated helpless.
However, if Hiram were paralysed, the blow would not change his condition, and Miriam would be free to swing again the next round, again causing 10-60% of Hiram's original maximum hit points.