DBNO (Down But Not Out) is a game mechanic in Tom Clancy's Rainbow Six Siege.
DBNO is a situation in which an Operator has lost all 100 points of health but hasn't died. When going below 0 health points for the first time in around, players have a 20 HP buffer during which they will enter the DBNO state instead of dying. In this state, the player will slowly bleed to death unless they are revived by a friendly player or hit by a Doc Stim Pistol. If the last shot to hit the player causes enough damage to take them beyond a 20 HP buffer, they will skip the DBNO state and die instantly. However, wearing Rook's armor plates remove this limit and guarantees a DBNO as long as it isn't a headshot.
A player cannot get downed when rappelling, and anytime the player enters the normal DBNO conditions while rappelling, they will die instantly. A player can get downed while climbing a ladder, in which case they will drop to the bottom of the ladder.
In this state of incapacitation, the Operator will be unable to stand or defend themselves and have to seek help from teammates before getting damaged again. DBNO players can move by slowly crawling along the floor or they can remain stationary and hold their interaction button to slow down the bleed out instead. The bleed time is the same for all armor values but doubled when wearing Rook's armor.
Excluding special circumstances, each player can only enter the DBNO state once per round. If a player was injured and revived, the next time he/she goes below 0 health points will lead to instant death.
Under most circumstances, if the last player on a team is downed, they will die instantly, granting their enemies the win.
If a DBNO player is manually revived, they will regain 50 HP. If revived by a Stim Shot, their HP goes up to 75. However, if the player is Zofia and gets up herself, they will get up at 1 HP.
Frost's Welcome Mats puts victims into an immediate DBNO state if they step on them. While trapped by a Welcome Mat, the victim can neither move nor slow down their bleeding.
The Welcome Mat and Caveira's Luison ignores the one DBNO limit - players can only get downed by their attacks, even if they were already downed before (though this does not mean that they can't kill DBNO players). However, players revived from them are still functionally identical to other revived DBNO players and will die if they suffer other forms of incapacitating damage.
Doc and Zofia offer exceptions to the rule. Doc is able to self-revive himself with his Stim Pistol, while Zofia has a special "Withstand" Ability, allowing her to recover from her DBNO state at the cost of regaining only one hit point.
The hostage has a slightly higher DBNO buffer than an Operator's and can go into a DBNO state. Though, it will take longer for the hostage to die since it's a stationary objective. This means it is possible to make the game go into overtime if the hostage is still in DBNO and hasn't died yet.
However, the DBNO state is skipped if a frag grenade or a headshot is achieved.