Active recall means retrieving something from memory rather than looking at it again. Closing the book and trying to say what was on the page is active recall. Rereading the page is not.
The effort of retrieval is the thing that strengthens the memory. Not the exposure, not the time spent, not the number of passes. The strain of pulling something out when it is not immediately available.
Rereading produces fluency. The material becomes familiar, it goes down easily, and that ease feels like understanding. It is not. What has improved is your ability to recognise the text, which is a different skill from producing the content when the text is not in front of you.
The problem is that the feeling is convincing and it points the wrong way. Rereading feels productive and is weak. Retrieval practice feels uncomfortable and difficult, which is the sensation of it working.
Highlighting has the same failure mode, with an extra one on top: it defers the thinking. Marking a sentence as important is a decision to engage with it later, and later usually does not arrive.
The common feature is producing the answer before you check it. The moment you look first, it stops being retrieval.
Active recall says retrieve rather than review. Spaced repetition says when to retrieve: just before you would have forgotten, which is when the effort is highest and the strengthening effect is greatest.
The two are separate ideas that work best together. A flashcard system without spacing means you drill things you already know. Spacing without retrieval means you see material on a good schedule and do nothing useful with it.
Answering before you have actually tried. Flipping the card at the moment you feel you probably know it, rather than after you have produced the answer, converts retrieval practice back into rereading and you lose most of the benefit.
The rule is simple and easy to break: answer in your head, out loud, or on paper. Then flip.
No. Active recall is retrieving rather than reviewing. Spaced repetition is the timing of those retrievals. They are usually used together.
No. Cards are one convenient format. Writing from memory, past papers and explaining aloud are all active recall.
Because it is difficult, and difficulty is a poor guide to effectiveness. The methods that feel smooth tend to be the weak ones.