Among teachers of iaido there are various approaches, two of them main, to when the head and eyes turn to meet an opponent when a form involves unsheathing the sword to turn and attack.
One main approach is to lead with the eyes. Some teachers emphasize that all iai movements should appear as natural movements from everyday life, thus when turning one should move as one does in everyday situations. Ishido sensei of Kawasaki city 'walked me through' an example. Having me stand in front of him and slowly walking forward he then said "Hey Chris!" as though he had recognized me from behind, as in two people walking down the street. As he called my name I turned to meet him as I would for anyone who called out my name from behind me. In this example I did indeed lead with the head and eyes. "That..." he said "is how you should turn in iai."
Similarly, Higuchi sensei of Yame city stressed the importance of proper eye contact and visualization during form training. "Without making the proper eye contact to target one has lost all visualization of the situation. Without this one is reduced to dancing." Hence, in turning the eyes again lead from the start of the turn.
The second main approach is to turn with the head and eyes but well after another movement in the form has taken place. For example, in Gammen ate, form number eight of the ZNKR seitei iai set, there are many teachers who emphazise that one must begin to draw the sheath of the sword back, then turn with the head and body at the same time. This also occurs in other kata such as tsuka ate, shiho giri, and ushiro.
These two main approaches are markedly different. One leads the turn with the head and the eyes, preceeding all movements. The other leads with the unsheathing of the sword followed by a turn of the head and body as a whole unit. Somewhere in between is a third version of leading with the unsheathing of the sword followed by a turn of the head and then the body in distinct phases. The first approach is again clearly differentiated from the second and third approaches.
As I have outlined above there is a very important reason for leading turns with the head and eyes: One must first see the enemy in order to accurately assess identity, position, relative distance and attacking stance.
So why is it that teachers like Mr. Yamamoto and Mr. Kamikokuryo teach leading with the second approach? Why would one lead by unsheathing the sword and then moving the head and body to directly face the target?
Kamikokuryo sensei responded to this question with "Kokoro no me'-'eyes of the heart.' In this approach the swordperson is not only 'aware' that an enemy is indeed behind him but has a relative idea of the enemy's position, distance and so forth. In this approach one may argue that, assuming the swordperson's judgment or intuition regarding the enemy from behind is accurate, the initial slow unsheathing of the sword can go largely undetected by the enemy. The butt end of the sheath faces the enemy. As it is slowly being drawn (while still facing forward) the enemy may not detect this because there is no lateral movement. Depth perception is more limited than lateral (in this case the butt end is being drawn towards the enemy. The same argumentation applies to drawing the sword towards the enemy's face from frontal positions: The enemy may not be able to accurately assess just how much of the sword has been unsheathed.) If the unsheathing goes unnoticed by the enemy the swordperson has created an advantage: In turning to face the enemy the sword is well into the unsheathing. Up until this point the enemy probably felt that surprise was within her domain. But as one can see, the swordperson about to turn to face the enemy has stealthily created an advantage by having the start of his counter attack go unnoticed. The enemy continues to come into the trap which has been set for her, unaware that the counter attack has already begun. In approach number one, leading with the head and eyes, the enemy is quickly aware that her attack has been noticed. The onus is then on the swordperson to quickly finish the turn and draw at once. One may argue that this open transmission of a counter attack would most likely lead to mutual, simultaneous strikes -- aiuchi.
Very recently I happened to find a small treasure of a read in which 'kokoro no me' can be solidly touched upon as a quality of the masterswordperson. D.T Suzuki in Studies in Zen talks about what he describes as the 'telepathic intuition' that pervades many famous tales of master Japanese swordsmen. Dr. Suzuki goes on to describe a story about Yagyu Takima-no-kami...
Yagyu Takima-no-kami was one spring day in his garden admiring the cherry trees in full bloom. He was, to all appearances, deeply absorbed in contemplation. He suddenly felt a sakki threatening him from behind. Yagyu turned around, but did not see any human being approaching except the young boy attendant who generally follows his lord carrying his sword. Yagyu could not determine the source from which emanated the sakki. This fact puzzled him exceedingly. For he had acquired after long training in swordplay a kind of sixth sense whereby he could detect at once the presence of sakki. (p. 23)In a footnote Dr. Suzuki goes on to describe the sakki. "Sakki literally means 'air of murder.' The swordsman frequently refers to this kind of incident. It is something indescribable, only felt inwardly as emanating from a person or an object..." and "The sakki also comes out of a person who harbors covertly or manifestly the idea of killing somebody. This 'air' is also said to hover over a detachment of soldiers intent on attacking the enemy."
We can see that this sakki and kokoro no me are by and large, the same concept. Some teachers note that this method of turning is an advanced one. Beginners should turn by leading with the head and eyes.
More than likely this concept applies to the first approach of leading with the head and eyes as well. In both methods sakki may be felt or, the enemy is simply 'heard' approaching from behind. The difference between the approaches is in the timing and transmission of the counter attack as I have outlined above.
Yagyu eventually discovered that his sword carrying servant, after seeing his lord so intently focussed on the beauty of the cherry blossoms, thought that even he could kill his master at that moment. Yagyu, realizing that his sakki had been accurate after all, did not punish the servant.