Half-Life · PC · Manipulation · Saves: Entire run time reduced by roughly 60% versus walking routes · Documented: 1999
Exploiting the GoldSrc engine's air acceleration model to maintain and accumulate speed through continuous jumping, allowing the player to move at several times normal walking speed throughout the entire game.
Half-Life's GoldSrc engine inherited Quake's fundamental movement physics with modifications that unintentionally made bunny hopping more powerful. In Quake, momentum preservation during jumps was constrained by how air acceleration was applied. In GoldSrc, the air acceleration model allows speed to accumulate more aggressively during strafe inputs while airborne: each airborne frame applies a small additional velocity in the strafe direction without a speed cap that would limit accumulation. Consecutive jumps with correctly angled strafe inputs — a rhythmic pattern that experienced runners execute with smooth, near-automatic timing — allow speed to grow well past the game's intended cap of 320 units per second, reaching 500, 1000, or even higher in long unobstructed corridors. This accumulated speed persists as long as the player remains airborne, making obstacle-free sections of the game traversable at extraordinary velocity. The accelerated back hop (ABH), a specific variant, involves holding the backwards key while jumping and strafing, which due to a sign error in the engine's backwards movement handling accumulates speed even faster than forward bunny hopping — making it counter-intuitively faster to move backwards through the game than forwards in certain situations. Half-Life speedrunning became one of the most technically demanding in PC gaming as a result, requiring runners to maintain optimal hop timing, strafe angles, and momentum through densely scripted areas across a three-hour game at sub-fifty-minute pace.
GoldSrc's air acceleration function adds velocity to the player each airborne frame based on the strafe input direction and an acceleration constant. The critical parameter is the speed cap applied to this function: the cap constrains how much speed the air acceleration term can add, but it is applied to the component of the player's velocity in the strafe direction specifically, not to the total speed magnitude. This means that if the player's total speed already exceeds the cap, the acceleration term still adds velocity at a reduced rate rather than being fully blocked.
By adjusting strafe direction each frame to stay just within the range where the acceleration term contributes positively, runners can achieve monotonically increasing speed. The optimal angle between the player's movement direction and the strafe input direction for maximum speed gain is approximately 45 degrees, which produces a spiral-shaped ideal movement path through open geometry. In practice, runners approximate this by feeling the acceleration feedback and adjusting angle rhythmically.
Half-Life's speedrunning community produced some of the most ambitious collaborative projects in the history of the medium. The Half-Life Done Quick series, beginning in the early 2000s, coordinated dozens of runners each optimising specific chapters, then assembling segments into complete runs that demonstrated the game's full exploitable depth. The 2014 Half-Life Done Quick in 20:41 video, produced by the SourceRuns team, became one of the most-viewed speedrun demonstrations in gaming history prior to the GDQ era.
Bunny hopping was the throughline technique in all of these records: without it, the game's extensive scripted sequences and narrow corridors would limit movement speed to walk speed for most of the run's duration. With it, the game becomes a physics puzzle — every room is evaluated for its bunny hopping potential, every corridor for its acceleration-sustaining length. The GoldSrc community's deep documentation of the technique's mechanics, written in the late 1990s and early 2000s on forums and text files, constitutes some of the most thorough player-produced physics analysis in gaming history.