A Sacrificial KO, as the name implies, is a knockout in the Super Smash Bros. series wherein an attack or maneuver executed by an aggressor leads to both the knockout of an opponent and the attacker as well. Sacrificial KO's are useful generally when the "kamikaze" has a stock or score advantage or when a match can be won outright by a sacrificial attack.
It is interesting to note that while some Sacrificial KO's are performed even when stock or score are even, they do present an opportunity to "even the score". Some players regard the sacrifice as worth performing even when the stock or score are even in order to reset the damage percentage if the character who performs the technique is at a percentage disadvantage. Other reasons may include vengeance or spite.
Notable examples from Super Smash Bros. Melee include Donkey Kong's ability to leap from a platform while grab-carrying, Fox's Firefox (aimed with no hope of recovery), and various meteor smashes executed too far from the edge of a stage. The down special move executed by Captain Falcon, the Falcon Kick, can also be used in this manner.
As competitive play generally has a "no holds barred" attitude against any manner of combos or techniques, a Sacrificial KO could be considered as a legitimate part of the arsenal of a player who holds to this consensus to win at all costs, no matter what others may think.