In essence, chakra is at its very core the power to connect.
By spreading the creed of Ninshū and by sharing chakra with the rest of humanity, the Sage of the Six Paths had intended for chakra to become a connecting force for good. He believed that if humans could truly come to understand one another, lasting peace could be achieved— for who would willingly spread pain, if they could understand the full extent of the suffering they would cause?
However, connecting was quite a broad term.
For instance, if you found out that your wife was connecting with your neighbour behind your back while you were at work, you could make your fist connect with your neighbour's jaw, which was arguably much easier than establishing a connection that would lead to mutual understanding (no, you may not touch my wife at this time).
And that is precisely what humans had done with the Sage's teachings, except on a much larger scale. Humanity ended up weaponizing chakra against each other, and Genjutsu was just one of chakra's many applications!
"As I was saying, you must first seize control of your opponent's chakra pathway network to initiate a Genjutsu," Kurenai explained. "This can be achieved by targeting any of your opponent's five senses."
To reiterate, chakra is at its very core the power to connect.
On a macro scale, this can refer to people interacting with each other, either peacefully or aggressively. But on a micro scale, chakra is the power that connects the body, mind and soul. Spiritual energy from the mind and physical energy from the body's trillions of cells come together to create chakra— but since everything is connected, the opposite can also become true.
For instance, Kakashi's Chidori isn't just about stabbing opponents with a fistful of lightning chakra. A crucial part of his Ninjutsu is the lightning chakra that he sends backwards through his own chakra pathway network, which allows him to consciously override his nervous system's commands to his cells. This allows him to surpass his muscles' limits, by making them forcefully contract or relax on command.
Indeed, the Chidori's greatest strength is not the unblockable stab, but the fulgurant burst of speed that it grants its user (which is why owning a Sharingan is highly recommended for this Jutsu's users, because of the tunnel vision effect).
"Once you've sent just a little bit of your chakra into your opponent's body via one of their five senses, you can begin taking control," Kurenai continued. "Your goal is to make your opponent's chakra resonate with yours, just like a tuning fork. You draw your opponent into the Genjutsu world you've created inside of your own head by copying it into theirs, and you gradually rob them of their sense of reality…"
Once again, it is very important to understand that chakra is the power to connect!
If you are opening your own mind to resonate with your opponent's, then you are obviously making yourself vulnerable to Genjutsu Reversal Techniques, in which your opponent seizes control of your own Genjutsu world and traps you within it!
"There are two schools of thought when it comes to Genjutsu," Kurenai explained. "The first is the stealthy approach, in which you avoid making any obvious changes that would make your opponent realize that they've fallen for your Genjutsu. Instead, you make extremely subtle changes to the world they perceive in order to gain an advantage in combat."
Kurenai looked thoughtful for a moment, as she thought of an example.
"For instance, you could make a low-hanging branch psychologically invisible to your opponent," Kurenai suggested. "Then, when your opponent bangs their head on the branch during your fight, you can seize the opportunity to defeat them while they're off balance."
It was the trickster's way of fighting, basically. The Genjutsu talent required to master this kind of illusion was quite low, but on the other hand, this wasn't the kind of trick that could finish off an opponent by itself. To neutralize your target, you'd still need to be proficient in either Ninjutsu or Taijutsu.
"The second approach is the one that I personally prefer," said Kurenai, puffing out her chest (A large C-cup was Sakura's guess) in pride. "Overwhelming your opponent's senses. I like to perform a total lockdown on my opponents' movements by tricking them into thinking that I'm using the First Hokage's Wood Style Ninjutsu."
Kurenai would generate an illusion in her opponents' minds of a tree binding their limbs with its branches and roots, therefore making them think they couldn't move in real life. Then, she could just stab them to death while they were immobilized!
"Oh!" said Sakura, who was very impressed. "So, you make your enemies think you're the village's secret weapon, or something? And since they're panicking, they never realize that they're under a Genjutsu…"
"Well, not exactly," said Kurenai slightly sheepishly. "There's never been another Wood Style user since the First Hokage, so the impact and fear factor has lessened quite a bit over the years. And there's also the fact that the Tree Binding Death illusion that I use is quite popular among the Leaf's Genjutsu practitioners, so the secret's been out for quite some time already…"
"Oh," said Sakura, who was now sounding disappointed.
Still, not everyone could perform Genjutsu Release— and depending on one's proficiency with Genjutsu, it was even possible to assume direct control over an opponent's very chakra. If you couldn't even control your own chakra, then there was no way for you to disturb the chakra flow in your head— meaning that the only way to escape such a Genjutsu was having a buddy disrupt your chakra for you… or accepting death.
So, to recap, Genjutsu was not totally useless, just very unpopular, since it was a rather roundabout way of neutralizing your opponent, at least when compared to Ninjutsu or Taijutsu. But because of its relative nicheness, Genjutsu was rather hard to deal with for the uninitiated… especially if the illusions were either hard to notice or extremely disturbing body horror.
Sakura's first two opponents in the third exam wouldn't be the easiest to deal with using Genjutsu. As long as Rock Lee used any of the Eight Gates, his chakra would go wild, making it impossible to control. As for Gaara, he was a Jinchūriki, meaning that he permanently had a buddy available to shake him awake. Theoretically, Shukaku would be able to give Gaara's chakra a nudge to help free him from any non-Mangekyō Genjutsu, but it remained to be seen if the One-Tails would actually help the one he hated the most…
"I want to master Genjutsu, Kurenai-sensei!" said Sakura defiantly. "Even if I can't make use of it in the short term, I believe that Genjutsu can become a powerful tool in my arsenal!"
"Very well, then," said Kurenai, nodding wisely. "In that case, let me teach you the hand signs for the Demonic Illusion: Tree Binding Death, and then you can test it out on Kiba."
Sakura glanced at Kiba, and a chill ran down her spine.
It certainly looked like Kiba was looking forward to some bondage play with her…