+50% Speed when a trooper drops below half of his life points.
When Life Point is less than 50%.
"I don't know about you, but I don't plan on spending eternity here!"
Survival Instinct is an unlockable passive Ability. Once the trooper's Health Points drop below 50%, his Speed improves by 50%. However, it's generally not too useful in PvP battles, as your trooper will most likely be dead before he can take advantage of it. It can make a difference for Soldiers, though.
It is an entirely different situation in an Exterminate mission though. If activated and coupled with Frenetic it gives a Trooper a small Initiative boost after every kill for the duration of the mission. This is especially useful for snipers with Munitions specialization.