Many warriors in the Warhammer world are not alive in the true sense, but are magical constructs bound to the will of a wizard.
The magic that empowers Unstable creatures is prone to ebbing and flowing with the tide of battle. The more heavily beset the Unstable creatures are, the more likely that the magic that binds them will fade away.
Unstable units that lose a combat suffer one extra Wound for every point by which they lose the combat, with no saves of any kind permitted against these wounds.
If an Unstable unit also contains Unstable characters, or is an Unstable character riding an Unstable monster, the controlling player first allocates wounds to the unit/monster, then divides any remaining wounds (if any) as equally as possible amongst the characters.
Characters that are not themselves Unstable are not permitted to join units that are (even if they become temporarily unstable for some reason).