No single Mandarin study technique works for every skill. Vocabulary, grammar, listening, and speaking improve through different kinds of repetition and feedback.
A strong study plan mixes methods instead of overusing one. Flashcards may help retention, but speaking drills build fluency, and guided listening improves recognition under real speed.
The useful question is not which technique is best in general, but which technique solves your current bottleneck. Once you identify that, your progress becomes much easier to manage.