There are multiple ways to jump cancel, I don't know how the numpad trick works so I'll explain the method I use
You want to hold down the jump key, release it and hit it again within the same frame. If you have shift bound as jump you can use both shift keys to make it more reasonable (or just bind 2 keys you're more comfortable with). The timing is very precise and takes a lot of practice and getting used to.
The reason why this works is because the game multiplies your vertical speed by 0.45 when the release_jump_button is triggered. Jump cancelling lets you release the jump button multiple times in a single jump and that's why a cancel looks so weird too.