Well, the Gates are located on eight specific points along our chakra pathway. Ordinarily they regulate the amount of chakra that flows through our bodies, putting a limit on our overall ability but maintaining the physical health of our bodies. Opening the Eight Inner Gates is like removing these restraints on the body, letting ample amounts of chakra flood the body and forcibly unlock one's full potential. For example, opening the First Gate (Kaimon) allows the user to access 100% of their physical strength as opposed to the usual 20%, by removing the restraints on the brain.
Sometimes, unlocking one of the gates also allows one access to powerful Taijutsu techniques that are usually too taxing on the body. For example, opening the third gate allows one access to the Front Lotus attack.
As you continue to open the Gates, your physical ability is is also periodically augmented; As one is continuously granted strength, however, the damage done to their body is also proportionally severe.
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The Eight Gates(As taken from Wiki):
The Gate of Opening (Kaimon): Located in the brain. Opening of this gate removes the restraints of the brain on the muscles, allowing the user to normally utilize 100% of their physical strength - This is significant, considering people can normally access a fraction of that at 20% of their total physical strength. Unlocking of this gate allows one to use the Front Lotus.
The Gate of Healing (Kyūmon): Located in the brain. Opening of this gate forcibly increases one's physical strength and temporarily re-energises the body, allowing one to fight for extended periods of time even after the stress of opening the first gate.
The Gate of Life (Seimon): Located on the spinal cord. Opening of this gate allows the user to use the Reverse Lotus. The increased blood flow turns the skin red.
The Gate of Pain (Shōmon): Located on the spinal cord. Opening of this gate increases the user's speed and power. May cause muscle tissue to tear on use.
The Gate of Limit (Tomon): Located in the abdomen. Opening of this gate increases the user's speed and power.
The Gate of View (Keimon): Located in the stomach. Opening of this gate increases the user's speed and power, as well as allowing the user to perform the Morning Peacock. Opening of this gate releases such enormous amounts of chakra that it can cause nearby water bodies to form a vortex around the user.
The Gate of Wonder (Kyōmon): Located below the stomach. Opening of this gate further increases the user's speed and power, along with allowing one to perform the Daytime Tiger. The sweat that comes off of people who open this gate emanates an almost unnatural green glow sweat. The side effects of opening this gate are that the user's muscle fibres are ripped to shreds, causing intense pain if anything or anyone touches them.
The Gate of Death (Shimon): Located at the heart. Releasing this gate uses up all of the body's energy. It makes the heart pump at maximum power and exceeds the power of every other gate. Opening this gate gives the user power that is even beyond the five Kage, but comes at the expense of the user's life shortly after.