To kiai or not kiai. A kiai (気合) is a shout or scream one emits some time during the technique. Ki 気 is one's energy and ai 合 means to bring together so from their definitions we can see that the kiai is supposed to bring together all of one's energy and spirit and is thought to be the pinnacle of one's power.
It turns out the use of the kiai can be backed up by science. Sounds like screams or kiai fall into an auditory spectrum that directly triggers the part of our brains that control the fight or flight mechanism. According to a recent study, "The higher the roughness, the scarier the sound, said people asked to judge the screams. The researchers also monitored brain activity in study subjects as they listened to screams and other sounds. Screams triggered increased activity in the amygdala, the region of the brain that processes fear response. Interestingly, when scientists manipulated non-threatening sounds to increase their roughness, the listeners' fear responses increased, as well, with more activity in the amygdala."
The kiai not only activates you as you summon all your power it but it can also trigger your opponent to lose his. In the history of Aikido and in some current iterations of Aikido there still exists the usage of the kiai. However today, the use of a kiai has evolved out of use.
One of the reasons that it has evolved out of use has to do with where one finds themselves in their training. Beginners need to summon all of their power to steel themselves and make the techniques work so there is an importance on "activating" oneself. As one evolves as a martial artist they come to understand humanity and therefore it becomes less of an emphasis on "activating" oneself and more of a need for us to be balanced, calm and steadfast.