Retrieval practice, as described in Book - Make it stick.. the science of successful learning: Recalling facts, concepts, or events from memory.
The book pulls from many studies to show that getting tested on material is one of the best ways to strengthen our memory by far. One example is a meta analysis from 2013, comparing multiple common studying techniques: dunloskyImprovingStudentsLearning2013
One explanation for this phenomenon could be that Learning is more efficient when it is effortful
Testing ourselves via retrieval practice also helps mitigate misconceptions about how well we know the material, see The illusion of knowing hinders us when studying
Reflection if another form of retrieval practice, so it follows that Reflecting on past performances makes improving at any skill way faster. Also, when the retrieval practice matches the later application of the problem, it is more effective (see: Practice like you play and you will play like you practice)
Using flashcards (Anki, Quizlet) to study means that one can get both the benefits of retrieval and those of spacing (see: Spacing out practice of a topic produces better learning)
Video - How to Study Using the Generation Effect $ Psych of Play by Daryl Talks Games illustrates this concept in the context of video games: Only reading the textbook and going to the exam ist like playing the tutorial and going straight to the final boss. Therefore, we need to test ourselves in order to create the "rest of the game". It also gives a concrete example of how recall appears naturally in video games: Almost every child that plays Pokémon knows the type chart by heart, which is impressive, considering we are speaking about a 18x18 matrix resulting in 324 possible attack vs defense combinations. However, they learn it in a highly effective manner, as every single attack is a retrieval problem on the attacks effectiveness.