Elijah listens to his student's response, which had quite a few things about Genjutsu, but they were very vague. Elijah already prepared to cover them individually, since it was his specialty.
"Not bad, though still lacking quite a bit of information. Nonetheless, we'll start with the most basic of all. How does Genjutsu work, really? Well, as you said, by disturbing the opponent's chakra flow. This allows the caster to let the opponent experience everything his five senses are capable of experiencing. Like tasting something through the sense of taste, hearing odd sounds through the sense of hearing, etc. You get the point. Nonetheless, to make a person understand it better, one has to look at the default chakra flow.
A normal shinobi's chakra flow is usually calm and balanced, when not affected by an illusion. A perfect balance between the physical and spiritual energies. Once an illusion is cast on someone, their balance between physical and spiritual energies are "scrambled" and disrupted, causing the illusion to take place. This is why those with Doujutsu or Sensory can see when their chakra flow is unbalanced, and then know its an illusion. A Genjutsu is usually induced through a certain trigger, which can vary from Sound, eyesight, hand seals or even light.
This also allows you to sometimes notice when an illusion is cast. And that's exactly where we're headed next. How can you detect that you are in a Genjutsu?"
Elijah asks, having spoken for a while now.