I watched a few episodes of handmade hero a few months ago, but got demotivated and haven't watched since then. What happens if I get back into it, watch tons of episodes, then stop, and forget everything? I've forgotten the talk about the cpu and motherboard in the C intro. I don't really want to re-watch hours of content just to get back up to speed. What should I do? Should I just skim over it, or re-watch it fully, or just skip re-watching it?

One place you might start is the episode guide, where you can search for specific topics you want to get up to speed on. I haven't watched handmade hero in it's entirety, but have previously used the episode guides to find parts of episodes which seem particularly worthwhile to watch.

For what it's worth, I think applying whatever knowledge you derive from the series is the best way to make it stick, whether by coding along or even better playing around with some of the concepts. It depends a lot on you individually, your level and goals.
I think approaching handmade hero from the view that you can watch it and learn how to make a game engine or learn system programming is a bit erroneous, not that it isnt a huge insight and source of information but the goal could maybe be understanding the ideas. Yeah a lot of information evaporates over time, especially if you dont use it actively, but ideas you understand and internalize have staying power, and kind of pull the facts and information once you start actually using it. As you go different concepts and ideas get connected in the ol' brain and you get "better" at programming and deriving the info you need quickly.

I mean the tldr should really be to skip the intro if you think you have a solid grasp on C. :p But whatever keeps you engaged for an extended period of time, think in months and years.

The best thing you can do is apply the knowledge you gain from Handmade Hero, like Halarious said.
I’ve also found Anki (or some other way to do spaced repetition) to be useful for when I can’t apply what I’ve learned immediately, but want to hold onto a concept.

An example of this would be how floating point numbers are represented. I don’t find myself using that piece of knowledge often enough in my day to day to be able to remember it reliably, but it is useful to not only have a conceptual model in your head, but also have the details on hand. Another example would be how lz compression works. I can certainly look up the specification as I need it, but it’s much more cumbersome than just being able to recall it from memory, which I would be able to do if I had applied the concept in practise.

Spaced repetition and flash cards are no silver bullet. Some things you just can’t turn into a flash card, and you always need to develop your conceptual understanding instead of relying on shallow memorisation. But, it does tend to make the more rarely used concepts stick in my head by periodically bringing them back to my attention.

I sort of got in the same boat, I followed along with a bunch of HMH a while ago and then got busy and didn't have time to continue practising low level programming.

VROOM, I would be interested if you were to elaborate on Anki and how you use it in this case!

Halarious
For what it's worth, I think applying whatever knowledge you derive from the series is the best way to make it stick, whether by coding along or even better playing around with some of the concepts. It depends a lot on you individually, your level and goals.
I think approaching handmade hero from the view that you can watch it and learn how to make a game engine or learn system programming is a bit erroneous, not that it isnt a huge insight and source of information but the goal could maybe be understanding the ideas. Yeah a lot of information evaporates over time, especially if you dont use it actively, but ideas you understand and internalize have staying power, and kind of pull the facts and information once you start actually using it. As you go different concepts and ideas get connected in the ol' brain and you get "better" at programming and deriving the info you need quickly.

I mean the tldr should really be to skip the intro if you think you have a solid grasp on C. :p But whatever keeps you engaged for an extended period of time, think in months and years.

I agree with this, find a way to internalize Handmade Hero in your own project. Take Casey's platform layer and write your own game, even something as simple as Astroids rendered in software would help a lot.

VROOM
The best thing you can do is apply the knowledge you gain from Handmade Hero, like Halarious said.
I’ve also found Anki (or some other way to do spaced repetition) to be useful for when I can’t apply what I’ve learned immediately, but want to hold onto a concept.

I am using Anki for a bunch of other stuff such as learning Japanese and Geography. However I have had some trouble applying this to programming. Do you have any tips for writing good anki flashcards for programming concepts and the sort of stuff you just have to know in Win32 and OpenGL etc?