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The purpose with the down up stroke involves timing. For the longest time I did not learn how important this was. I just picked what I thought sounded cool. For me it made it hard to play guitar with other people who had learned how to stay in time. Now I try to practice any new music piece using a metronome. This helped me understand the importance of the down up strokes. If you are trying to learn a new song and you can’t stay in time it very hard to learn with any consistence. If you work to develop this it will make it much more enjoyable and you can improve your playing more quickly. If this is all new it will be frustrating but will pay off if you develop good fundamentals. Just start slowly and it will come to you.