How do I do it?

First, you need to make sure you'll fully against a wall. Once you've done that, jump and then shoot your force ball on the very next frame. Good luck!

What GameGuy is talking about is known as an Omega Boost, btw. It's used when there's no corner of blocks to boost off of in the direction you want to go.

You can also (where appropriate) stand fully against a corner of normal blocks and simply fire the force ball and you'll still get boosted up and in the opposite direction.

There's many variations to the strat overall, I encourage you to continue watching runs and practising all the different variations. We're here if you need any more explanations. :D

thanks. im still trying to get frame perfect. Ill keep trying!

Yeah that's no easy trick. I used to try to flick my thumb through the jump and weapon buttons to get the timing, but now I instead place my index on the jump button and my middle on the weapon button (all this on controller btw) and then try to commit the timing to muscle memory.

You'll get there! :D