Mine Usage
Mines are "anti-notes," entities that punish the player if the corresponding key is held as it passes the receptors.
Because the player needs to avoid the mines, they are perfect for most situations where it is preferable to force the player to release a key. These can be placed at abrupt changes in the music, such as silences in the music or wherever particular instrument tracks stop. This is the most literal usage of mines. They can be used for making notes "pop." By placing triggers under notes, the players can be forced to lightly hit them in order to immediately release the keys after striking them, as if the fingers were bouncing on the keys. This is most often associated with staccato notes.
Another common usage of mines is almost a sort of "delayering" technique. The idea is that as a song transitions from one section to another, a change in layering might be too abrupt or unwanted, so mines are used to fill in the layers that would normally have notes, but do not use notes since they would clutter the chart or make it too dense.
Yet another alternative is to use the mines for particular effects. If there are a lot of background effects that the notecharter wants to highlight, but cannot afford to do so using notes, mines can be a good alternative. Other notable effects can be highlighted with mines as well, such as tremolos/pitch bends and filters.
Mines can also simply be used to draw pictures. Drawing particular shapes or symbols using mines can be something to keep the player interested during sections where nothing is happening, to bring to attention something very specific, or even as comedy. However, do be aware that someone playing the opposite scroll direction as you are is going to see your picture upside-down. Historically, within the StepMania/Etterna community, most all files use imagery recognizable from upscroll, but by default, assume downscroll, as it is the more common general VSRG scroll direction.
There are mines scattered throughout the chart, but they are most notable in the chorus, where there is a wide range of instrument sounds that are abruptly cut off, and this is highlighted by the mine usage following the note attacks.
The mines in the yellow sections are used as "delayering", following background sounds that do not require the player to hit in order to maintain focus on the melodic guitar. In the green sections, we can see the mines being used to simulate the vibrato effect, as the mines are placed in locations that line up with the ends of the pitch variation.
Mines are easily abused because just the presence of the mine alone has a very strong psychological effect on the player. Seeing the mine immediately draws attention to it since it sticks out, usually causing the player to try too hard to evade them. In addition, because mines will activate as long as the key is held down, care must be taken not to place them too close to the notes, making them feel very tight and forcing good accuracy and absurdly quick lifts for those notes. As such, moderation with mines is important, and they should be taken into consideration when balancing the difficulty of a chart.