To attemptt to chop of a foe's limb the following condition may need to be met:
- Must begin the round in melee combat distance from foe (no running in an hacking off limbs).
- Must have either won initiative or be grasped by the limb of the foe you wish to sever.
- Or have a foe that is completely surprised
- Or have a foe being held or captured in some manner.
- Must have an edged weapon heavy enough to chop off the limb in question.
The attacker makes an attack roll at -4 to hit (or attack with disadvantage for you new fangled rules uers).
The damage from the blow must be 5 or more points of damage (or optionally 4+ 1 per 2 HD/levels of foe. Some monsters already have special hp totals for body locations and some special attacks these suggestions don't necessarily trump the rules for specific monsters.)
The foe if hit and the damage roll is high enough the foe is still allowed a save vs death (or a reflex save as appropriate for your game). If the save succeeds the blow was mostly avoided and only 1/2 of damage is suffered. If the save fails the limb in question has been severed.
Having a limb is severed is traumatic, most foes will attempt to break off combat if they have enough sense to do so but some with multiple limbs or driven by bestial rage may not pause long enough to notice.
There you go a combatant has the choice of normal attack or attacking with a lesser chance to hit and a fair chance of causing less damage.
further notes on severed limbs:
This is bloody business, until properly bandaged 1hp a minute will be lost from mortal foes that can bleed.
Healing magic will stop the blood loss.
Creatures that regenerate, are constructs, supernatural, or undead may be able to simple reattach a severed limb as appropriate for type of monster.
Some monsters are just begging to get the occasional extra limb hacked off. Why bother having four-armeed carnivorous gorillas, octopus men, or five headed dragons if nothing is going to get lopped off?