Propclipping is a glitch in new engine Source games where if you push a prop against yourself, it will get stuck inside you. When you save and load with a prop inside you, you will be pushed downwards through geometry thanks to Saveload Clipping.
In Source 2004 (and in some Source 2006 mods), J-Clip can be used to get stuck with props instead.
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In new engine, propclipping is most consistent by picking up a prop, and having it be pressed against the wall as flat as possible. The results of propclips are notoriously random due to their high precision finicky.
Some props work better than others, these include but are not limited to: an item box, a barrel, metal folding chair, etc.
Example of the baby clip formerly used in Half-Life 2 RTA runs:
https://www.youtube.com/watch?v=l8wBzITjIEA
Being pushed against a prop by a door:
https://www.youtube.com/watch?v=tu3TQNOA2js
Black Mesa propclipping works almost identically to new engine, but with a higher degree of consistency due to the players high speed while crouching, allowed by enabling "always run" in the settings. The best angle to look for most Black Mesa propclips is -69°.
Propclipping using a barrel:
https://www.youtube.com/watch?v=QralZKJD3u0
https://www.youtube.com/watch?v=nG2josZzvA4
TODO: this whole section cause i dont know the technical details of it -eold