The Eight Gates

The Eight Gates release the body's limits one gate at a time, trading health for overwhelming speed and power.

Learning the Gates

Learned in the Taijutsu tree: 8 Gates (level 10, 3 points, Taijutsu 25, requires Leaf Whirlwind and Might Guy's Approval, which is used up). The approval comes only from beating Might Guy in the Trial of the Eight Gates, and he gives it rarely.
Learning it gives you the "8 Gates" item.

Opening and Closing

Right click (with the 8 Gates in hand or selected on the loadout) opens the first gate.
Stage Up (Y), while holding the 8 Gates item in either hand, opens the next.
There is no stepping down ("The gates only open further").
To close, sneak, look straight down (more than 60 degrees) and right click. This works from any gate but the eighth ("There is no closing the Gate of Death").
Opening the gates ends any other body mode.

The Gates

Gate 1: Gate of Opening. Level 10.
Gate 2: Gate of Healing. Level 20.
Gate 3: Gate of Life. Level 30.
Gate 4: Gate of Pain. Level 40.
Gate 5: Gate of Limit. Level 50.
Gate 6: Gate of View. Level 60.
Gate 7: Gate of Wonder. Level 70.
Gate 8: Gate of Death. Level 85.

Mastery

A gate opens only once the one before it is mastered by time spent inside it:

Gate 1: 900 seconds (15 minutes).
Gate 2: 1,500 seconds.
Gate 3: 2,100 seconds.
Gate 4: 2,700 seconds.
Gate 5: 3,300 seconds.
Gate 6: 5,100 seconds.
Gate 7: 6,000 seconds.

That's six hours in all. The tree tooltip shows the gate being mastered and your progress, and "X mastered: the next gate can open." appears when you finish one.

Power and Cost

Movement speed: +20%, +40%, +60%, +85%, +110%, +140%, +180% and +280% for gates 1 to 8 (the eighth nearly four times as fast).
Attack damage: +6 per gate number (+6 at gate 1 up to +48 at gate 8). It counts for bare-fist blows as well as weapons.
Health loss: every 4 seconds you lose a share of max health: gate 1 1%, gate 3 1.5%, gate 4 2%, gate 5 3%, gate 6 4%, gate 7 5%, gate 8 7%. Gate 2 instead heals 5% every 5 seconds. Not in creative.

Visuals

Steam rises off the body from the first gate.
From the third gate, an aura burns round the body: green, brighter green, lime, fierce green, cyan at the seventh, and blood red at the eighth.
From gate 5, opening a gate blasts out a wind explosion.

Gate Techniques

Morning Peacock (gate 6, right click)

8 second cooldown.
1.5 seconds of punches so fast they catch fire: 40 homing fireballs a second (60 in total) at the target you aim at within 40 blocks, 7 fire damage each.

Daytime Tiger (gate 7, right click)

10 second cooldown.
The air is drawn into the fist for 0.6 seconds, then loosed as a roaring tiger projectile that bursts in a 7-block wind explosion: 120 force damage to everything within 7 blocks, throwing them away.

Evening Elephant (gate 8, attack key)

2.5 seconds between uses.
Five air-cannon punches, one every 0.25 seconds, each a crushing wave: 45 force damage in a 9-block, 35-degree cone, plus a push.

Night Guy (gate 8, right click)

You are wrapped in a crimson dragon of steam (1 second forming, held still), then fly into a kick along your look at high speed for 1 second.
It deals 250 force damage once to everything it meets (with a fire explosion on each), with explosions along the way.
Then the gates close, and your body slowly crumbles to ash over 8 seconds (heavy Slowness and Weakness, dust) and you die.

Closing the Gates

Closing the gates gives Slowness II and Weakness I for 10 seconds.
After the eighth gate has been opened, the gates stay shut for 10 minutes.