They practice writing correct for-loops, branches, recursive functions, efficient graph searches, input-output, data structure uses etc. in a very fast pace. To be able to do that requires you to chunk a significant amount of information.
I found that doing stuff very fast correctly makes you learn and internalize concepts quickly and more deeply.
This is all unfortunately anecdotal, I'm not sure how to google for such research (not just in cp but in "learning to do stuff fast improves learning rate").
Yet every instrument I've played and tried to learn, the moment I tried to play stuff fast but correctly (be it drums or piano), I had to improve my technique, had to internalize the rhythm patterns (more complex rhythms are insanely difficult to do fast), memory etc. With it came a significant amount of progress.
Same thing happened to me with language. I was speaking English for 20 years but still had trouble with fluent pronunciation (despite my writing feeling natural), I knew what I had to say but somehow my tongue got all tangled. Yet, when I tried learning some rap songs that use insanely fast diction, my speaking improved up to a point where it felt normal.
On the other hand, something like Project Euler or even Advent of Code is very nice, if you do it at your own pace, or for learning a new language.