With this optional rule, when a character is reduced to 0 HP, instead of falling Unconscious and beginning to make Death Saving Throws as normal, a character can instead choose to take one Final Stand.
When this rule is invoked, the character immediately stands up and may take a Full Turn, ignoring the current Initiative Order, with all of their resources available to them. They gain their full Movement, Action, Bonus Action, and Reaction, as well as the use of any expended class features or spell slots they might have already previously used. Additionally, all actions a character makes during their Final Stand are made with Advantage.
Once a character's Final Stand turn is completed, they fall to the ground dead as though they had failed three Death Saving Throws.