"Aiki" is actually an old name for a group of kenjutsu tactics that employed mental inertia, disruption and involuntary reflex to defeat ones opponent. These concepts can be purely applied in taijutsu as aiki no waza (aiki jutsu) although it is a much less decisive tactic in taijutsu than it is in an engagement utilizing edged weapons. In taijutsu, aiki principles are most effectively employed in conjunction with jujutsu waza ... hence the name aikijujutsu.
At its highest level of execution aikijujutsu is characterized by sophisticated jujutsu waza which employs mental disruption and soft joint locks to throw or immobilize an attacker. Aiki taken even farther in taijutsu becomes the aforementioned aiki no waza (aiki jutsu) which almost totally eschews joint locking in favor of very subtle kuzushi and mental disruption to defeat an attack. Most aiki no waza should be viewed as a study of physical and mental dynamics as opposed to effective self defense. The extreme level of intricacy required for the effective application of aiki no waza make it a risky proposition in an art like taijutsu where even a successfully thrown opponent can recover and attack again. However, aiki jutsu (principles) applied in kenjutsu can be startlingly effective as the momentary kuzushi or mental disruption resulting from the application of "aiki" can be immediately followed by a life ending sword cut.