Jack Theory
A jack is a pattern where the player must hit notes on particular column(s) repeatedly, in quick succession.
The whole premise of jack theory is that jacks can be used to manipulate the level of tension in the patterns being used. By effectively applying this concept, you can turn "weak" patterns into "strong" patterns.
There are a few reasons why jack theory is an important technique. First of all, it's extremely versatile in terms of what it can be used for. Secondly, it expands your options in terms of the possible patterns you can use. Last but not least, it gives your charts a lot more depth and complexity that otherwise would be difficult, or maybe even impossible, to accomplish.
Here is an example. Suppose you have one beat in the chart that is split into 16th notes. The first 16th note is a kick drum, the second 16th is part of some background rhythm, and the third 16th is a snare. If you were making a 4k chart and you used [12] for the kick and [34] for the snare, you'd get a weak pattern that barely emphasizes both sounds. However, if you did [12] for the kick, 1 for the next note, creating a jack on the column 1, and then [34] for the snare, you'd get a stronger pattern. The kick drum and jack keep the left hand grounded, while pressure is built up. When this pressure is let go, the [34] snare hit feels like a stronger attack. This brings out both sounds, rather than just hitting them separately, which doesn't really distinguish the sounds as well.
The [12]2 pattern builds tension in the left hand that is released on column 3, making the upbeat pop out and emphasizing the subsequent syncopated rhythms with the renewed attention to the drum groove. (The rest of the chart makes use of some very heavy jack theory, both literal usage as well as more abstract flow control.)
Limiting your pattern choices by avoiding jacks can limit your work for higher difficulties, where one might be charting something fast and/or wants to be fancy and more complex, especially for 4-key, where you don't have a lot of room to begin with. By incorporating jacks into your patterns, you not only have more options, but these jacks will add a lot of expression into your work. Mastery of jack theory will put a lot of personality into your charts, as well as making your patterns stronger and more dynamic.
A jack theory masterclass. Notice the various different ways that jacks are used throughout. Some are literal, following a particular sound or layering, while others are there to manipulate the texture. Either way, the careful positioning of the notes to form jacks exactly where they are desired or not shapes the back and forth tension/release dynamic, whether that dynamic is reinforced by the melody, drums, or some other instrument. This draws your attention to different instruments through the song despite the patterns maintaining relatively consistent layering structure.