J-Clip (Jared-Clip), is a glitch in Old Engine Half-Life 2, which allows the player to get stuck by inserting a prop into the upper half of their body. After that, the player is able to use Saveload Clipping in order to clip up. It is not possible in Episode One.
Before J-Clip can be used, it has to be activated.
In order to activate the J-Clip state, the player must Saveload, while fully crouched.
Because of HL2 containing an auto-crouch-jump mechanic, saving and loading right after jumping works as well.
after the player enters the J-Clip state, they will be in it until they saveload while not crouched.
Entering the J-Clip state grants the player the ability to put props into the upper half of their body. This is because the props still think that the player is crouched, and will only collide with the player as if they were crouched.
https://www.youtube.com/watch?v=wzkIvbWYQYU
In order to start Saveload Clipping with J-clip, the player must put a prop inside of their upper half, start crouching, and then start saveloading. Do note that after the loading finishes, if the player doesn't pause, the prop will simply fall out of the player's hands.
In scripted speedruns, the SPT command spt_pause 1 is used, which pauses for the player automatically. It is very useful when J-Clipping.
This is however not all that's needed to perform J-Clip most of the time.
In order for the Saveload Clipping to work, the player must have velocity when they first get stuck with J-Clip.
This is usually done by walking forwards right before smashing the prop into the player's head and starting the saveload chain. Many players also jump right before saveloading, to increase the chance of the clip succeeding.
https://www.youtube.com/watch?v=awuhxeSGxVY
If the player pauses on every load when J-Clipping, and is standing against a wall, the prop will often travel up together with the player, up until the celling, assuming it is not too high. The distance that the player is able to travel upwards depends on the prop, but usually is only about 6-7 saveloads.