It'd be easy to assume the GIF above is the most impressive part of this run, but it's not. This incredibly fast speedrun of Half-Life 2: Episode Two was pulled off by SourceRuns Team, and they managed make it through Valve's shooter in record time: 30 minutes and 36 seconds.
In order to make this work, some parts of the run weren't done consecutively, so that particular strategies could be employed:
Timing starts when the crosshair appears and stops when it disappears. Segmented means that the run was done in multiple parts so riskier and faster strategies could be used. The version of the game is from 2009, build 4104. Some scripts (such as a jumping and duck spam script) were used.
One of the first things you'll notice is how most of the speedrun takes place backwards. Here's their explanation why that's so useful:
Short version: If you jump backwards you go fast.
Long version: ABH or Accelerated Back Hopping is an unintended side effect of fixing bunny hopping. If you're not familiar with the 2004 version of Half-Life 2, every time you jumped forwards with W held down you gained a little bit of speed. This could be exploited by jumping immediately after landing; you could gradually gain speed.
Then people at Valve decided to fix bunny hopping for Episode 2 and Portal. So how the fix works is if your horizontal speed is greater than the speed cap you get accelerated the amount you're over the limit backwards. This gets applied every time you jump. The obvious way to exploit this fix is to simply turn around so that your speed is increased, instead of decreased.
The speedrunners actually break down how they pull off every glitch and exploit moment-by-moment in an exhaustive and super interesting document that makes for fascinating reading.
God damn, this makes me want Half-Life 3.