"A death strike is used to kill an unconscious or immobile victim. You must touch a weapon to the torso of the victim while saying "Death strike one, death strike two, death strike three". This verbal must be spoken clearly and at a normal speaking pace. You do not need to have skill with the weapon to inflict a death strike."
It is important to note that a death strike can't be used on creatures or characters who can still move about in some way. If they are able to use their weapons or writhe out of the way of attacks - even if they must stand in one place while doing so - then they're not vulnerable to a death strike. From the Accelerant core rules:
"To inflict a death strike, the victim must be unmoving. The victim can be unconscious or unmoving because of a game effect, but you cannot inflict a death strike on a body until it comes to rest after an attack, nor can you inflict a death strike on a helpless but struggling victim. If the victim can move, the victim can prevent a death strike. First you must render the victim unconscious or unmoving, and then you may inflict a death strike."
This does not mean that the character is doomed once a death strike is initiated:
"To interrupt a death strike, you must strike the person attempting to deliver the death strike with a melee, missile or packet attack. You may also interrupt the death strike by striking the weapon used to perform the death strike with a weapon of your own. You do not have to force the weapon away. A death strike that is interrupted is canceled and has no effect."
It is possible to keep attempting to death strike a character after being chased away, but the count must be started over again - it can't be picked up where it left off.