Flow and Texture
Flow or texture describes the feel of a continuous stream of notes. Suppose you have two single note stream patterns with the exact same rhythm and speed. They could potentially feel very different, and therefore should be used differently. For instance, 123412341234 feels very different from 142314231423. Even though they are both repeating a sequence of 4 notes that uses each column once, the first pattern feels like it is more smooth, while the second pattern feels more sharp. This has to do with how the hands alternate note attacks. Notice that in the first pattern, hands only alternate every 2 notes, whereas they alternate every note in the second pattern. This tells us that simply shuffling one pattern into another is not necessarily a good substitute, and that the positioning of the notes matter.
Here is a straightforward example, where the contrast between two sections is very simple and pronounced. The yellow section shows how a pattern can flow smoothly, because the notes move from side to side like a wave. The red section shows a more jarring pattern, the alternating of notes between each hand, making each hit feel deliberate and punchy.
This concept is particularly useful to highlight and match the different textures in the music. Some specific examples of where this can be important include when you have multiple layers with juxtaposed rhythms, such as with regards to syncopation or polyrhythms. Placing the exact rhythms of each layer may result in neither rhythm coming through, due to the way the notes blend together texturally. Being cognizant of these pitfalls makes sure that your chart is not only trascribed accurately, but also feels like it fits the music.
There are two layers in the simple highlighted section: the synth melody and the clap. Notice that whenever the clap sound appears next to the synth sound, they are always placed on alternate hands from each other. This makes the distinction between the two different sounds as clear as possible, and that the rhythms don't blend too smoothly into each other.
Flow can not only be affected by alternating hands, but also jacks, holds, mines, and more. Experimenting with patterns and playtesting is the best way to familiarize yourself with the different possible textures at your disposal. Of course, this concept doesn't imply that in order to achieve a particular texture, you have to, for example, stick to a very rigid form of constant hand-alternating notes. It just means this is a property we can pay more attention to so we can be aware of why a particular pattern works or doesn't work with a part of the music.