I know if you enter and use the eight gates formation that Guy Sensei used, you will die. So I was wondering, is there a way you can use it and won't die? Perhaps use it for a second or a really short time and get out? Or not even use, just enter it for showing? And also how does one know he can use the last gate?
The 8th Gate, also known as The Gate of Death (死門, Shimon), located at the heart, requires the user to stab their chest with their thumb to direct the chakra to the tenketsu. Once opened, the user will enter what's known as the Eight Gates Released Formation. Opening this gate will cause the user's blood to emanate a flaming aura of red steam, the so-called Steam of Blood (血の蒸気, Chi no Jōki) from all pores of their body, hair, and eyebrows. Releasing this gate uses up all of the body's energy while making the heart pump at maximum power. Exceeding the power of every other gate, the user is temporarily granted approximately a hundred times their normal power, far beyond that of the five Kage. Once activated, those who are capable can perform the Evening Elephant and the Night Guy. The side effect of opening this gate is that it comes at the expense of the user's life, causing the user to crumble to ash after their chakra runs out, having virtually cooked themselves from the inside-out. This is unlike opening the other gates, which simply injure or damage the body.
I don't think you can live after using the Eighth Gate because the Jutsu user has to thumb crush his heart and follow the blood from the user's heart directly.
I think to open the Eighth Gate you have to limit the overall flow of chakra within a person's body. The basis for the idea of the chakra gates comes from the body's limits on the functions within it. This makes the body much weaker, but it keeps the body from expiring too soon.
It is heavily implied that the User has no control over stopping what it does. How it kills you was never actually stated, but it is implied that the gate can not be closed and while open consumes chakra constantly. This is however an interpretation of what was shown, when
Guy himself used the 8th gate against Madara Uchiha as the 10 tails jinchuriki. Upon piercing his heart with his thumb, he opened the 8th gate. Once he used his last attack, he was left in critical state and otherwise completely incapacitated, but it was also shown that his heart was still consuming the last little bit of chakra he had left. The other side effect also includes Literal incineration, As guy was practically on fire from the inside, and his blood was boiling. His final attack turned his toes to ash, and it looked almost like lava was coming out of his body.
It is also known that you die if you use all of your chakra, which has happened in the series, as shown when
Kakashi died saving Choji in his fight against Pain. He exhausted all of his chakra to deal with the missile and died. It is also implied through dialog that Nagato also died due to chakra exhaustion, when he used Rinne art of Rebirth to revive Kakashi and the others he killed.
So, Its quite reasonable to assume that, since unlike the other gates, opening the 8th gate requires injuring yourself, there is no direct way to undo that action and close the gate. Since while it is open it seems to constantly consume chakra, and can't be closed, it will eventually consume all of it, which kills you. Later on, we find out
there is a way to stop it. How it was stopped was not explained at all, but Naruto, who had obtained the Senjutsu of the Sage of Six paths, was able to use Yin-Yang release and somehow closed the gate, saving Guy's Life. Guy was permanently crippled already however, and Naruto could not heal that damage. So somehow, Naruto was able to heal guy, and undo the process that no other shinobi could undo.
So, Without some extreme circumstances, there is no way even remotely possible to turn it off. Activating the 8th gate is a death sentence, and the only clue as to how to turn it off requires the legendary abilities of the Sage of Six Paths himself.
So, pure speculation here based on the stuff in the spoilers, but
Perhaps, if someone like Naruto, who obtained the Yin-Yang release healing abilities, also learned how to open the 8th gate, he might be able to activate it and then close it. The Yin-Yang healing based techniques Naruto acquired, at least temporarily, were capable of even creating a new regular eye for Kakashi when he lost his Sharingan one. However, the side effects of the 8th gate are severe, so even if only being activated for a short time, he would be temporarily crippled upon deactivation.
lastly, As for how they know they can activate it, it is not truly identified. All we know is that the main way is to pierce your heart with your thumb, which somehow activates it. Considering it kills you upon activation, the body would probably not let you do it. However, it is still just a gate, So it would make sense that it would work similarity to the other 7. Likely, something forcefully prevents the gate from opening, a seal of some kind. Piercing your heart could then either be destroying that seal, which causes the gate to open just as easily as any other gate, or it might physically destroy the gate itself, effectively opening it, but otherwise it cant close since it was destroyed. Since it is just a gate though, Its probably quite obvious to the user that something is blocking it from opening. Guy hinted that he might have found out how to do so from his father, and we know Lee knows how to do so since Guy revealed how its done. There may also be a scroll somewhere that teaches you how to do so, or at least up to the first 7 and records of what happened with the 8th gate.