Jclip
From SourceRuns
Jclip is an Old Engine technique for getting stuck in props in order to clip out-of-bounds. It is achieved by crouching with a prop, saving the game, loading the save, standing up and moving the object inside of the player; the player can then repeatedly save and load while crouched to head upwards.
Currently this technique is only viable in RTA runs and in conjunction with Source Pause Tool, an Autohotkey Script is also recommended.