Energy lost via Quick Thinking utilizes the Warframe's armor value. The efficiency value will act as an additional damage reduction with the values listed above.
Works on self-inflicted damage.
As of Update 11.1, Quick Thinking will keep your minimum health at exactly 2 units and use your remaining energy to keep you alive from incoming damage. The efficiency will go up with the mod's level.
Rage will still grant you energy taken to your health until you reach 2 HP (filling up your energy pool), at which point the mod will stop giving energy (as you are not taking health damage) and Quick Thinking will take over, using your Energy pool as a buffer against your remaining 2 HP. Once energy is depleted, you will go below 2 health and die.
While under the effects of Quick Thinking (2 HP and losing energy instead), strong hits against you will cause a 'Stagger' effect, briefly causing your Warframe to stumble in place. Be wary, as repeated strong hits can effectively stun-lock you in place until you die. Weak hits against you will not induce this effect.
If a single attack is sufficiently strong enough to kill in one strike, the player will enter bleedout immediately and not lose any energy. This is to avoid wasting energy, as the player would've been killed regardless.
The 14.6.0 fix resolves issues where Rage + Quick Thinking gave effective permanent invulnerability because energy was being added to pool that was never dealt to player. With this change in mind, the combination of Rage and Quick Thinking may seem less efficient as before due to no longer giving effective invulnerability. The scope of this change is being monitored to ensure the usefulness of these Mods as a combination.
Increased stagger threshold and made stagger only occur on big hits instead of on accumulated damage; this should prevent situations where Quick Thinking would save you only to let you get knocked down and killed again.