Accelerated Back Hopping

Accelerated Back Hopping (ABH for short) is a glitch in OrangeBox engine games that is used as a main movement method in speedruns. ABH replaces Bunnyhopping which was previously used in Old Source Engine games and fixed in the OrangeBox engine. ABH is also significantly faster than Bunnyhopping.

Instructions

Jump forward, turn around in the air and jump right as you land. It's generally recommended not to hold S or W(or your respective keys for moving forwards and back) while ABHing. Binding jump to mouse wheel or using a jumping script is recommended as it makes timing jumps a lot easier.

Explanation

When you exceed the game's speed limit, the game tries to slow you down whenever you jump, back to the desired speed. By default the game thinks that you're moving forwards, so when you exceed the speed limit, it'll accelerate you backwards. If you are facing backwards, this will only increase your speed. So, the faster you're going - the more you will get accelerated. The speed limit varies: it's 209 (165 in Portal) UPS while crouched, 225 while walking, 285 while running and 352 while sprinting. It's easiest to reach the speed limit while you are crouching.

Variants

There are some variations in the execution of the glitch, that allow easier navigation through the levels.

Accelerated Forward Hopping (AFH)

When you gained some speed while ABH-ing, you can turn around and hold S. This way the game thinks that you are traveling backwards and so it will try to decelerate you by applying force the direction you are facing, so you still gain speed. But it is not as fast as ABH because when you are holding S, you are slowing down due to airstrafing. So when you AFH it is best to hold S only when you hit the ground and jump. This is not true in Portal, you can hold S the entire time without much penalty because of the lessened air control.

Accelerated Side Hopping (ASH)

You can also ABH sideways. To do that you need to jump, turn your mouse a bit to the side and then hold S + strafe key opposite to the direction that you turned your mouse to (if you turned your mouse to the left, you'll need to hold S + D, to the right - S + A).