This team effort from SourceRuns isn't your average speedrun, but that's what makes the way they blow through Valve's classic all the more entertaining. Via PC Gamer, it's a run that makes use of an old 2006 build of the game, one that has different physics and movement values to the 2016 model.
This run was done on an older version of Half-Life 2, more specifically, a build from 2006 that still ran on the original game engine (also known as the Old Engine), before being updated in 2010 to the OrangeBox engine.
Old engine has significant movement differences, as well as a long list tricks and glitches now patched. The most notable are bunnyhopping and prop-flying.
The tricks that allowed us to save the most time were triggerdelaying (also present in Half-Life 1) and our version of save-buffering. Both of these tricks are exclusive to this engine, and were discovered fairly recently (in 2014).
It soon became apparent that using this version would allow us beat the game in a much shorter time than we've ever expected!
Here it is:
If you're wondering how they do all those clipping jumps, here's a video explaining it:
As you can see, in the hands of people who know what they're doing, Half-Life 2 is transformed from a brooding FPS into a Mirror's Edge x Tony Hawk's experience, all running along walls and surfing on busted old wooden palettes.
And when I said it was a team effort, I wasn't kidding. The 40-minute video is actually a compilation of 506 "segments", weaved together into a single run.