Rereading notes feels productive because recognition is easy — seeing familiar material creates a sense of fluency that gets mistaken for actual learning. Research by Henry Roediger and Jeffrey Karpicke, in a widely-cited 2006 study, found that students who studied material once and then took a practice test retained significantly more a week later than students who reread the same material multiple times, even though the rereading group felt more confident about their mastery immediately afterward. The act of retrieval itself — actively pulling information out of memory, rather than passively recognizing it — appears to be what strengthens the memory trace.
This is called the testing effect (or retrieval practice), and it applies even when the "test" is just self-quizzing with no grade attached: closing the book and trying to recall key points from memory, then checking what you missed, produces measurably better retention than reading the same material an equivalent number of times. The discomfort of struggling to recall something is often a sign the technique is working, not a sign you don't know the material — which is exactly why it feels less pleasant than the false fluency of rereading.
Roediger and Karpicke had college students study short passages, then split them into groups: some reread the passage multiple times, others read it once and then took a practice test recalling as much as they could, with no feedback given. Immediately afterward, the rereaders reported feeling more confident about their mastery of the material. But tested again a week later, the single-read-plus-practice-test group recalled substantially more than the repeated-rereading group — the group that had felt less prepared right after studying ended up remembering more when it actually mattered.