It allows the user to perceive motions. An example of this was in the battle between Sasuke and Haku. Sasuke could see Haku's attacks coming. The user's pupil turns bloody red, and gains an extra pupil, or tomoe within each eye that will surround the original, centered pupil. When active the user now has the ability to track speeds that normally cannot be seen by the regular human eye. They also gain the ability to see through visual D rank genjutsu and can predict most taijutsu and ninjutsu movement and technique, allowing one to be able to react best to the situation.
This is what you can do. Now we will activate your 2nd Tomoe and further training.
We will training in 2 sessions:
First training session: The student will begin with his/her Sharingan activated. The Sensei will be focused into teaching Sharingan's tracking abilities, by hiding himself/herself in diferent terrains such as at the bottom of the Lake, between trees deep into the forest, underground, in a Rocky Environment and dessert. The student will have to find his/her sensei's location using only his Sharingan, by seeking out chakra signals, heat source, the slightest moves.
Second training session: The student needs to be trained in the ability to see through visual D rank genjutsu and to predict most taijutsu and ninjutsu movement and techniques. Sharingan Sensei will explain how genjutsu works, using D rank examples which the student will have to pierce through and then the sensei will make fast handseals and fast taijutsu attacks. The training will end when the student will be able to predict/see through at least 2 handseals and 2 taijutsu attacks.
After these are completed your 2nd tomoe will be activated. Understood ?
