You seem to be talking about streams.
According to your profile, you have only started playing osu! for about 2 months and played less than 200 maps. This means you're still a beginner. At this stage, you should just play more games and you'll learn eventually.
However, I recently managed to teach it to a friend who also just started playing osu!. I don't know how much of a difference it made that I could teach it to him face to face, but I'll try to write it down as good as I can.
What I'm trying to convey to you is a basic piano hand position. On one hand, that's because I have years of experience playing the piano, so I'm familiar with it. On the other hand, when playing the piano, you'll occasionally come across trills. These are played pretty much exactly as streams in osu!, bar the cursor movement, so it fits that as well.
I'll write down 3 steps that should help you learn that hand position.
In the first step, I'll try to introduce you to the finger movement. We'll not use the keyboard yet. Place your hand on the table and make sure that it has room to move around, at the very least 30 cm forwards. Now, have you ever simulated a walking person with your index and middle finger? Imagine that your hand is a person, and the index and middle fingers are the persons legs. I encourage you to vary the speed, try slow walking as well as the fastest you can manage.
In the next step, we establish the hand position. Try to form your hand as if it was holding an apple, and place it on the table so that the finger tips of your index and middle finger are orthogonal to the table. You can use this picture as a reference:
https://upload.wikimedia.org/wikibooks/ ... lavier.jpg You can rest your wrist on the table, though - while you'll move your hand during this step, it should still work fine on the table. Now, as in the first step, use your index and middle finger to walk your hand around on the table. Try to maintain that apple-holding hand position, and as in the first step, try varying the walking speed.
The last step is the final adjustment for osu!, so now take the keyboard and position your index and middle finger on their key. Go into the hand position that we established in step 2. Now, you'll basically want to 'walk on the spot' - it's exactly like step 2 but without moving your hand.
Not that you'll always want your fingers to be in a relaxed state. As a piano player, I'm well aware that relaxed fingers are more rapid as well as more accurate. However, I'm aware that this might not come trivially if you're not used to it and actually might take a bit to adapt to, so keep that in mind.
I don't know if that changes anything, but I fully-alternate. It might be that this is suboptimal fur single-tapper, but since they're forced to alternate in streams as well I guess it doesn't really matter.
