AssaultCube is a frame based game so everything you do will be done when the next frame is being handled.
Let's assume you got 100 fps which means that every action you do (like pressing a key/button) will be handled every 10 milliseconds.
So when you are able to press space and shift in 10 milliseconds you'll archive the same effect as if you would archive if you would bind it.
The lower the frame-rate is, the easier it is to perform a h4x jump without pre-jumping and bind.
Why is that? What happens when I press space or shift or something?
Well basically in this case when you want press a key to jump it sets a flag internally which indicates that you want to jump. Note the flag will only be set if you don't crouch at the moment.
So as soon as the next frame will be handled it'll make you jump. The same thing will happen when you press a button to crouch.
So if you do both actions (crouch and jump) at once with one key. A flag that indicates that you want to crouch and a flag that indicates that you want to jump will be set.
As soon as the next frame will be handled both actions will apply. At first it'll say ohh this players wants to jump so let's say he jumps. Then it'll say ohh the player wants to crouch so let him crouch. Since the second action which is crouching kind of overwrites the first one you stay on the floor and crouch. So nothing special happend yet.
What do I need to do to h4x jump out of this situation?
To make a doublejump/h4x jump out of this situation we need to:
- release the crouch button because as I mentioned before we can't jump while we crouch
- strafe run
- do all this in 250 milliseconds
So why have I pressed the key to jump and crouch again?
Well basically you've done it because you don't want to make a pre-jump and you can only h4x jump if your last jump was made 250ms before your current jump. That's why you needed that "jump".
Can I get any higher than h4x jump without rifle jumping?
Yeah for sure you can still crouch in the air to get even higher. But I'm not sure if h4x jump already refers to doing a doublejump + crouch in the air.