On this page:
These units have some sort of protective devices or supernatural vitality that may allow them to survive an attack that would kill another creature.
To represent this, if you fail an armour or cover save for a unit with Invulnerable Save (or if the unit doesn't have an armour or cover save) then you may attempt a special second save of 6+.
You may take this special second save against any form of attack, even attacks that would normally disallow saves. It always succeeds on a D6 roll of 6 — no modifiers ever apply.
FAQ
Q. Can invulnerable saves be used by units destroyed by dangerous terrain?
A. No.
Q. Can invulnerable saves be used by units destroyed by the extra hits from the result of an assault?
A. No.
Can invulnerable saves be used by units destroyed from ending their second withdrawal move within 15cm of the enemy?
No.
Can invulnerable saves be used by units destroyed from an additional hit from a Blast marker on a Broken formation?
No.
How many invulnerable saves must a unit make if it is hit by a Titan Killer weapon that causes multiple points of damage?
One, unless it is a War Engine in which case it must make one invulnerable save for each point of damage.