From Halopedia, the Halo wiki
The alert carry, sometimes mistakenly called "ceasing fire", "standing down", or "gun down", is a feature introduced in Halo 2 for machinima purposes, and has since appeared in Halo 3, Halo 3: ODST, Halo: Reach, Halo 4, Halo 2: Anniversary and Halo 5: Guardians. The feature allows gamers to holster or lower their in-game weapons; it is commonly used when making machinima. The exact execution of the alert carry has changed between games and requires different button combinations for different game controller layouts.
Halo: Combat Evolved
The alert carry can be performed in Halo: Combat Evolved by simply looking down. Although the player is looking at the ground, others will see their head looking forward with their arms and weapons pointed at the ground. Generally, using an M6D Pistol or a Plasma Pistol resulted in a more relaxed effect; using a Needler or an Assault Rifle tended to bend the left arm inwards.
Noticing that Red vs Blue, which frequently used the alert carry effect, helped the Halo franchise, Bungie made it easier to perform the technique in Halo 2 by assigning it its own button—down on the D-Pad.
The alert carry is shown by looking down at the player's legs, they will not be as bent as normal. Other players will see it as your weapon being lowered. Shooting, throwing grenades, meleeing, reloading, entering vehicles, dying, or pressing the D-Pad again will end the effect. On Xbox Live, the trick can also be performed by exhausting all grenades and holding L (Left Trigger).
In Halo 2, not all weapons can be put into ready position, such as dual-wield weapons, Rocket Launchers, and Energy Swords. The trick seems to work on two-handed rifle type weapons, as when the Brute Shot is lowered, but on closer inspection, the player's arm is bent slightly and the Brute Shot itself is slightly tilted.
Halo 3 and Halo 3: ODST
The Halo 3 alert carry was first performed in public by Red vs Blue cast members. In Halo 3, the feature is activated with a button combination. The player can cease fire by pressing and holding the following in any order (different according to button layout):
After holding the combination together for three seconds, a player will find that their weapon has disappeared off of their screen; the weapon is lowered and visible to other players. Some weapons, however, are barely or not at all affected by this trick; such weapons include the Energy Sword, Brute Shot, Gravity Hammer, Sentinel Beam, Plasma Cannon, Machine Gun Turret, and Flamethrower. The trick looks odd when performed with the M41 Rocket Launcher, Fuel Rod Gun, Spartan Laser, and dual-wielded weapons.
The animation of the Rocket Launcher, Fuel Rod Gun, and the Spartan Laser will look similar to that of the assault rifle in Halo: Combat Evolved (unless the player uses an Elite model, then there's no change). If a player is dual-wielding, looking around will desynchronize their hands.
Shooting, throwing grenades, meleeing, going into Edit Mode on Forge, dying or re-entering the button combination will end the effect.
The alert carry option returned in Halo: Reach and was first seen being used in the Red vs. Blue Halo: Reach PSA: Deja View. The player can alert carry by pressing and holding the following in any order (different according to button layout):
After holding the combination together for three seconds, a player will find that their weapon has disappeared off of their screen; the weapon is lowered and visible to other players. Like in Halo 3, players will be unable to perform it on Xbox LIVE.
Unlike Halo 3, the M41 Rocket Launcher, Fuel Rod Gun, Gravity Hammer, and the flag will be in a resting position after using alert carrying.
Unlike previous games, when the player alert carries while holding a detached turret, the camera angle will shift a bit and the Spartan will also appear to be slightly more relaxed.
Pre-release screenshots revealed that alert carry had been incorporated into Halo 4. However, it is only available in default control layout, though once activated, it will work in other control layouts. The controller input is to perform it is:
As in Halo 3 and Halo: Reach, alert carry doesn't work in Xbox Live games. Unlike the other games, alert carry only works when the player is standing still; any other movement will cancel the alert carry for as long as the player is moving.
Halo 2 Anniversary
The alert carry returns in Halo 2: Anniversary, fixing Halo 4's problem; in addition, the function can now be performed on Xbox Live. The particular combination of buttons used to enter alert carry varies depending on the selected control scheme, but corresponds to the following actions:
Unlike previous entries, dual-wielded weapons, the Brute Shot, and the Sentinel Beam can be alert-carried, and Elites can exclusively alert-carry the Machine Gun Turret. Note that switching your weapon will exit the Alert Carry stance.
Halo 5: Guardians
Alert carry in Halo 5: Guardians can be achieved with the new Machinima control setting in Custom Games. When activated, Hold Reload and "down" on the D-pad.