Propclipping is a glitch in New Engine Source games where if you push a prop against yourself, it will get stuck inside of you. When you save and load with a prop inside of 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.
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.
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 -69o
Todo: this whole section cause i dont know the technical details of it -eold
Half-Life 2 - Extravirus' example of the baby clip formerly used in RTA runs: https://youtu.be/l8wBzITjIEA
Black Mesa - Tmob propclipping using a barrel: (0:16) https://youtu.be/nG2josZzvA4?t=16