eye

Sharingan was a visual jutsu. Users put their own chakra into their eyes,
and ensnared opponents in the jutsu. In other words, as long as Itachi could
see the fight, he could ensnare his enemy in the jutsu.
Eyes meeting was not something that happened only subconsciously.
Expecting no particular trouble, an enemy would let their gaze race around
the scene. As long as that optical axis and the optical axis of the jutsu user
were in alignment, it was possible to create a situation in which the enemy’s
gaze would intersect with the jutsu user’s, without the enemy’s even being
aware of it.

Book of Dark Night Chapter 4 Part 2