Quickclip (Or QC) is a glitch in HL2-based games which allows the player to quickly clip through any wall, prop, or displacement that they want, by utilizing the mechanic of the player's Bounding Boxes.
Quickclip can be divided into 2 things. One, The Quickclip State, and Quickclipping itself. the Quickclip state allows quickclips to happen, and needs to be activated first.
That being said, It is not the only way to make quickclipping possible. Limited Quickclipping can be done by utilizing Item Save Glitch instead of the Quickclip state.
Having the QC state will also allow the player to Saveload Clip through anything, in any direction.
Do note that the quickclip state has may cause some unforeseen consequences, such as getting stuck in elevators.
The player can get rid of the Quickclip state by entering, and then leaving a vehicle or by Save Deleting.
Some objects in fanmade mods can sometimes be "quickclippable", where the player can just quickclip through the object without activating anything.
Quickclipping is significantly more consistent in Old Engine, but it is still possible in New Engine
The Quickclip state is activated by first achieving Yesclip, and then touching a barnacle. this will cause yesclip to disable, and quickclip to enable as the "noclip" property is lost.
In order to perform a quickclip the player needs a physics object.
In order to clip through a floor, the player can either stand on the physics object, walk into it, or simply put it inside of themselves, and Saveload Clip. Having the quickclip state makes physics objects not collide with the player, meaning that they can simply enter the player.
The saveload clip method will not cause any fall damage, while the other two generally will, but not always. Falling when standing on an object that the game considers "Heavy" will result in no fall damage.
Clipping through walls is generally harder.
The first method, is to simply hold a physics object in front of the player, then jump into a wall with it.
This is notoriously inconsistent in New Engine, where the 2nd method is usually used instead. This is because the prop can sometimes move inside of the player when clipping like this, which will result in the player losing all of their velocity in NE, but not in OE.
The 2nd method gets rid of this issue, as in it the prop is never held by the player.
To execute it, simply drop the prop in front of the wall that you want to clip through, and Accelerated Hop into it.
When TASing, it is possible to clip through walls faster by using Saveload Clipping.
If the player starts saveloading just as they enter a physics objects while in the quickclip state, they will retain their velocity and start clipping in its direction.
At high velocity this allows for very fast clipping.
By dropping a prop when travelling above around 700ups, the player will slam into the prop and QC forwards. This is the most consistent way to clip through walls when holding a prop, and it is the only consistent way to Quickclip with ISG.
In Old Engine, the quickclip state can replace getting stuck with J-Clip to clip up walls with a prop.
Simply put the prop inside of yourself, hold it against a wall while crouched, and saveload with spt_pause 1. The saveloads cannot be 0 ticks.
This is a lot more consistent in Source 2006 due to the game giving the player less velocity when clipping in a prop.
By putting a prop inside of the player while they're falling in Old Engine, the player will continue gaining downwards velocity.
Usually in OE it is only possible to clip through brushes from below, but since the quickclip state allows clipping from any direction, when the player saveloads with a lot of downwards velocity from gravity, they will clip down.
https://youtu.be/oM0F450StXw
The quickclip glitch is caused by the player's VPhys Bounding box losing collision.
There are multiple ways to achieve this.
Once the collision is gone, when the player touches a physics prop the Havok bbox will try to take priority. This means that the player will teleport to it as long as it is not inside of geometry or Out Of Bounds.
Because of that, it will continue travelling through everything until either the player stops touching the physics object, or until the havok box enters an empty space, at which point the player will warp to it, completing the quickclip.
Since the quickclip state cause the Havok bbox to not have collision, props can simply enter the player in the quickclip state.