When your shields hit zero you get a short window of invulnerability instead of dying — and building around that window, not around health, is how you survive Steel Path.
Shield gating is the brief window of invulnerability you get the moment your shields hit zero: any damage over what was left in the shield gets ignored instead of spilling into your health, and you can't be hit again until that window runs out. It's the reason a build with no health mods can still walk out of a room that one-shots everyone else — and if you've ever wondered why you die instantly in one mission and shrug off a rocket in the next, this is almost always the difference.
While you have any shields at all, an attack that would drain them can't also touch your health — the excess gets discarded at the shield line. The moment shields hit exactly zero, you enter the gate: a fixed stretch of time where you take no damage from anything that would normally hit your health, shields, or both. Move, shoot, revive an ally, whatever — you're untouchable until the timer runs out, then it's back to normal rules.
That timer isn't the same for everyone. Since the Abyss of Dagath update (October 2023), the length of the gate scales with how many shields you had at the moment they broke: the patch notes put it at a 0.33-second floor and a 2.5-second ceiling, reached once your shields are at 1,150 or higher. A Warframe sitting around 325 shields lands close to the old 1.3-second value the mechanic used before the rework.
Before that October 2023 rework, the size of the gate depended on whether your shields had fully recharged before they broke — a full recharge gave the long window, a partial one gave a much shorter one. That pushed players toward the opposite of what you'd expect: running the smallest possible shield pool, often with the Decaying Dragon Key, so shields would recharge fully in a couple of seconds and the long gate would be available constantly. Digital Extremes closed that loophole by making the gate scale off shield amount instead of recharge state, and capped what the Decaying Dragon Key can do to 0.33 seconds flat, so it no longer works as a shortcut to permanent invulnerability.
None of this matters if you only trigger the gate once per fight. The builds that hold Steel Path content are built to break their own shields on a loop:
Health regenerates slowly and rarely at all without specific mods or arcanes; shields recharge on their own after a few seconds out of combat, and a handful of abilities refill them instantly. That makes shields the resource worth spending, and health the resource worth never touching. A frame built around cycling the gate can eat hits that would delete a health-tank build outright, because it's never actually absorbing the damage — it's just not there when the hit lands. That's also why "why am I so weak" so often has nothing to do with your weapons or mod capacity: it's a frame with high health and low shields, taking full damage on every hit with no gate to fall back on.
(If you're also hearing about Tennokai and wondering if it's related — it isn't. That's a separate melee combo system layered onto stances, not a survival mechanic, so it won't help with any of this.)
A handful of things ignore it outright. Toxin damage bypasses shields entirely and hits health directly, so it never triggers a gate and can kill through one regardless of shield size. Damage types and sources that target health directly rather than shields work the same way. And the gate itself only ever protects against the next hit after shields hit zero — if you're still at zero shields when the window ends and haven't recovered any, the next attack goes straight to health with nothing standing in the way.