The Half-Orc class on PHB page 41 has a trait which states the following:
Relentless Endurance. When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest.
But what exactly is meant by not getting killed outright? Can you only use this when the damage is ongoing damage that puts you at 0 hit points? When exactly does this trait activate?