ABH is the ability to jump backwards and gain incredible, unnatural speed.
Hl2 and Eps, Portal 1 - NOT Portal 2, L4Ds, TF2 or Counter-Strikes
Gmod has been patched.

ABH requires three things, jumping, ducking/crouching and moving backward.
The method to achieve this effect is to move backwards, jump, duck in midair and continue to duck, then spam jump repeatedly.
Spamming jump whilst ducking (holding CTRL) and moving backward (S) is extremely hard, so to get around this we will bind jump to the mousewheel.

Bring up the console with the tilda or backquote key (~), (`), the key to the left of the 1 on your keyboard. If this doesn't bring up the console you need to enable it in the options by going to: Options, Keyboard, Advanced..., Enable developer console and try again.
To bind jump to scroll we need to enter these commands individually:
Do not worry about having jump stuck to these keys, if you want to unbind them in the future all you need to do is type:

If you think that you'd not want to override scroll wheel because you're using it to change weapons and not using numbers, you may prefer to use a script instead and bind to any key.
You can choose to make a macro using your keyboard software if it is supported, or you could get a script from the internet.
Here is a link to many[wiki.sourceruns.org]
This script uses a macro utility called AutoHotkey[www.autohotkey.com], so you will need to download that first.
These scripts are easy to edit and fairly straight forward.
The linked scripts have binds for several abilities: Jump spam [Space], Wallclimbing (Propflying) [C], and Ladderspeeding Tab. You can toggle this script using F7.

The original ABH script has been removed - the replacement link should be solid, but the binds vary across the individual breakdown and the final joined script. Ctrl (crouch) will also need to be held in addition to the jump spam script.

A brief explanation of how to edit these scripts:
The line "*f7:: Suspend, Toggle" toggles the script on and off, change f7 to whatever key you want to pause the script.
Other lines: " *Space::" "*CapsLock::" ";*Tab::" ";*LButton::" are the inputs for their respective macros.
Notice some lines have semi-colons behind them? These are comments. If you want to disable a single macro just put semi-colons behind the input you choose.
"{blind}" keeps Alt/Control/Shift/Win down if they started out in the down position.
The other curly brackets contain the other keypresses generated by the macro.

If you do not have space bound to jump in-game, ctrl bound to duck, e bound to pick-up then you will need to edit the inputs.

This trick needs an initial amount of speed, you can't begin whilst ducking, and you can't begin without some speed behind your first jump.
You can begin by facing backward, looking away from the direction of travel. Then do these things:

• 1. Move backwards (S)
  
• 2. Jump (SPACE or MWHEELUP/DOWN)
  
• 3. Hold Duck/Crouch (CTRL)
  
• 4. Stop holding backwards (let go of S)
  
• 5. Spam jump (Scroll up and down repeatedly)

*steps 2 and 5 can be replaced using the script macro

• You can turn into this trick by facing forward (towards where you want to go,) jump, turn, crouch and spam jump.
  
• Sprinting helps gain speed. Sprint into your initial jump to help you start off. Or you can continue to hold it whilst ABH'ing for even more acceleration. (I would advise to hold CTRL with your pinky and SHIFT with your ring finger over trying both with your pinky)
  
• You can gain some control (turning) by turning with your mouse, but this will only redirect you once you land a bounce and can be very hard to control.

Did you know the tau cannon on the buggy has a secondary fire that's much more deadly than the primary?
You can jump past (without ABH) the first physics balance puzzle in the game, skipping it entirely.
The pistol used to have a powerful secondary fire where it could charge and unload many bullets at a time.

Half-Life 2 Done With a High Magnitude of Velocity is a segmented world record speed run for (new engine) HL2, which uses this technique almost constantly. Skip to 41:40 for an instant insight into the possibilities.

There is now a faster speedrun using the old engine, which supports bunnyhopping (achieved by just constantly jumping,) flying, and differing save-load glitches - but not ABH.