Any time a player enters a vehicle in a Half-Life 2 - based game, they are given a few proporties.
The player's collision group is set to 10, they become immune to bullets, and they gain noclip, which usually cannot be felt, as the player is parented to the vehicle.
The Yesclip Glitch refers to a situation where the player manages to leave the vehicle without having these proporties taken away. This is usually done by having a Vehicle be deleted while the player is still inside of it. The Yesclip state refers to the state where the player is outside of a vehicle with all of the vehicle proporties, listed earlier.
Gaining yesclip is very situational and depends on the map, and engine.
If a vehicle is deleted by map logic while the player isn't in the exit animation, the game will crash.
Additionally, in Old Engine, the game will always crash if the vehicle is deleted by map logic, even in the exit animation.
In NE, Yesclip is usually achieved by using map logic deleting vehicles, and mapping oversights on map transitions.
As an example, we'll use the ending of the first cutscene in d3_breen_01.
There, the game forces the player into exiting a prisoner pod (which is a vehicle), after which it deletes the pod with game logic.
Using Saveload Lag, the player is able to delay the ending of the animation, allowing them to still be in the pod when it get's deleted. This causes the player to be freed and achieve the yesclip state.
Doing this in Old Engine will crash the game.
The 2nd method revolves around trigger_transitions. A trigger_transition is a part of a Map transition, and it is responsible for deciding which entities get transported over to the next map. If one exists, anything inside of it will get transported, and anything outisde will not.
In certain situations caused by mapping oversights, it is possible to have the whole vehicle be outside of the trigger_transition, exit the it, and instantly land in the trigger_changelevel.
This will cause the next map to instantly load, causing anything outside of the trigger_transition to be deleted. Since in this scenario the player is still in the exit animation, and the vehicle is outside of the transition trigger, Yesclip occurs.
Doing this in Old Engine is impossible, as triggers cannot be hit in the exit animation.
Getting yesclip in Old Engine utilizes either Save Warping or Changelevel Delay.
By saving inside of a vehicle which will not get transported to the next map, then warping with that save, the vehicle is deleted and the player achieves yesclip.
Similarly, with CLD, the player can simply drive through a changelevel trigger with the vehicle, and move away while CLD is happening so that they're outside of the transition trigger. Once the transition happens, the player and the vehicle will be too far away for the vehicle to be transported, yesclip.
In both NE and OE its possible to achieve yesclip by entering a changelevel trigger and entering a vehicle which is about to be deleted at the same time.
In order for this to be viable, fps needs to be lowered to a low value, like 30, or alternatively, sv_alternateticks can be set to 1. Modifying this command is however not considered legitimate in speedruns.
https://www.youtube.com/watch?v=J1ZVAf9Ex3k
There are 2 ways to get rid of Yesclip.
One, is to simply enter a vehicle again and then leave it. This will return the player to a state of normalness and remove the vehicle proporties.
The other is to get grabbed by a barnacle. this will cause the noclip property to get removed, but the other ones will remain. This is the cause for Quickclip