I’ve been dissecting this piece of code as well and it might be of some help if we could know what it is you want to do and what problem you are facing.
If you make your own Custom Physics mode then I think you need to set it with StartNewPhysics() after you’ve switched the MovementMode to your custom PhysMode.
If you are overriding PhysFalling and want to modify velocity you should be able to tweak velocity as you see fit and set it right around the part where they calculate their velocity, probably after or rewrite how they calculate velocity. Depends on what you are trying to do.
If you happen to have more insight then I please let me know as I’m trying to figure things out myself.