Dynamic difficulty is a gameplay mechanic wherein the difficulty of a game is automatically changed (either up or down) based on the performance of the player. In many cases, this can serve to make the game easier for a player who is struggling with a particular section, to allow them to progress past the difficult part. Similarly, some other games may raise the gameplay difficulty if the player performs well, in order to provide an added challenge.
Dynamic Difficulty in Shoot-'Em-Ups
Many shoot-'em-up games include some sort of dynamic difficulty, referred to typically as "rank." A typical instance of a rank mechanic may involve things like making enemies fire more bullets, making enemy bullets travel faster, or giving enemies increased durability. The specific things that control the rank often vary from game to game, but common things that will raise the rank include picking up a lot of power-ups, playing for a long time without losing a life, and so on. The most common way to lower the rank is typically to lose a life.
Difficulty Management as a Gameplay Element
A subset of shoot-'em-up games, such as many of those developed by Raizing, use the presence of a dynamic difficulty as a central gameplay concern that the player has to purposefully manipulate in order to complete the game. For example, in Battle Garegga, if the player simply plays the game in a straightforward way without taking the rank into consideration, the game will become essentially impossible at a certain point. By carefully avoiding doing things that will raise the rank and by strategically doing things that will lower the rank (such as purposely losing a life), the difficulty of the game can be kept at a reasonable level.
DoDonPachi Daifukkatsu Black Label makes this concept more explicit by placing a visible rank meter on the screen, which the player can raise and lower through specific ways. When the meter is high, it becomes much easier to rack up high scores, but the game becomes far more difficult. When the meter is low, survival is much easier, but the capacity for scoring is very low to match.
Dynamic Difficulty in Strategy Games
Some Strategy games, such as later Super Robot Wars games, rank players based on their performance in stages. Performing well, or accomplishing difficult optional objectives, will make the game harder, but will come with some sort of additional reward. In SRW's case this is often a bonus mission at the end of the game. Poor performance will correspondingly make the game easier to complete and lock out the additional reward.