Prop Clipping
Prop Clipping is a Glitch in which a physics prop gets stuck inside of the player. The glitch allows the player to Save/Load buffer into brushes, props, or OoB space.
How to
Pick up a prop, then walk into a wall. If you continue pressing yourself into the prop, the prop will usually clip into you. Once you are stuck you can Save/Load buffer down into the brush, prop, or OoB space below you. Alternatively, you can get the prop stuck in between two props so that it loses its collision for a short period of time. Then you can simply walk into it and wait until it regains collision.
Notable uses
List of places where this trick can be used.
Half-Life 2
d1_trainstion_04 - Skipping the cutscene introducing us to Alyx.
d1_canals_03 - Skipping most of the map.
d1_canals_11 - Skipping the cutscene where the gun is attached to the airboat, negating the need for taking the airbot in No Save Deletion runs.
d1_town_05 - Skipping the cutscene with Alyx and Leon.
d2_prison_01 - Skipping the gunship fight.
d2_prison_08 - Skipping the cutscene where Gordon and Alyx confront Judith.
d3_c17_01 - Skipping the cutscene, that is the whole map.
d3_c17_10b - Skipping the gate that has to be opened by grav-punting the orbs out of their containers.
d3_c17_13 - Skipping the cutscene with Dog and Barney at the end of the map.
d3_breen_01 - Skipping the cutscene with Alyx after you've regained control and the elevator ride down.