In the top left portion of the screen is the DMW or Digital Mind Wave. This mechanism is what can let you use a limit break, a summon, a special attack, grant status effects, and level up your materia along with Zack. These are all done through the spinning numbers and character portraits that act like a slot machine. When certain numbers or portraits line up they can activate any of the previously mentioned effects. Each section will explain one part of the DMW.
Your total SP, or SOLDIER Points, are listed in the top bar of the DMW. You gain SP by defeating enemies and you can also convert materia to SP in the Materia section of the main menu. Do not confuse SP with EXP; they are completely different. You can use your SP for Materia Fusion and rolling the DMW reels. Ten SP will automatically be deducted from your total each time they start spinning. If you do not have ten SP then the reels will not spin.
Your limit meter is the likelihood of the character portraits matching which will active the modulating phase. This bar can be affected by story events, causing the bar to be “Low”, “Normal”, “High”, “Sky High, or “Heavenly”. The higher the likely-hood the faster the frequency waves will be. The color will change as well to match this. Brighter colors equal higher likely-hood while darker colors equal less likely. If you go into modulating phase and use a limit break then the limit meter will go down a level each time. The limit meter can go up and down.
The DMW reels spin like slot machines. So when they stop they can sometimes grant certain effects depending on what they stopped on. When the first and last character portrait reels stop and match, combat will cease and you will enter into modulating phase. In this phase the center portrait reel and all three number reels will continue to spin until they stop. Matching three portraits gives you a limit break, while matching a certain combination of numbers either levels up your materia or Zack.
While the reels are still spinning you can enter into a special mode. You can either enter Summon Mode, Chocobo Mode, or Genesis Mode. These are activated randomly, but having more portraits unlocked on those reels can increase your chances. Theses modes switch the current reel with one of the other two reels and all three portraits begin to spin again. Genesis Mode will activate the same way, but works differently from the other two reels. While in Genesis mode the Genesis portrait will replace the characters that were previously on the ends and he will be added to the center reel as it continues to spin. He will remain on the reels for the length of the mission, chapter, or till you die.
While in the main menu you can select DMW. This will show all the possible character portraits along with each characters limit verge, limit progress, and how many times you have used that limit break. A limit verge happens when you go into modulating phase with that character. When that happens you are more likely to see memories you have with that character. The more memories and pictures you see of that character the more progress it will unlock. This will increase your chances of using a higher-level limit break.
The character reels do nothing while in combat until you hit modulating phase. Once you’re in modulating phase the main goal is to line up three characters to hit a limit break. This is completely random and can’t be modified. It seems to hit limit breaks more often if you have more progress and more characters unlocked. Even if you have not unlocked the character yet, three matching silhouettes will initiate a limit break.
A character portrait will sometimes appear next to your limit meter. This means that you are more likely to match reels with that character. This usually happens after a major story event.
The level of the limit break seems to be determined by the certain reel number or the non-matching number if two are matching. Usually all sevens with a limit break will be level 5. There does seem to be a little randomness to this aspect as well though.