The Combo mechanic is where a movement is activated within a game by pressing a certain sequence of buttons in a certain order. This relies on the skill of the player as they have to remember these sequences to complete the action and have to do so in a short amount of time.
This is mostly used in ‘Beat Em Up’ Games, also known as fighting games. A good example of a game that has lots of use of this mechanic is Mortal Kombat. The player can do basic punches and kicks with the buttons but by completing a sequence of the buttons it will perform a special attack that does more damage to the enemy.
Another time this mechanic is used is in a game called Party Panic. This is a game compiled of short mini games. In one of these games the players participate in an eating challenge. In order to eat more food than your opponents, you must press the sequence of buttons that are displayed on the player models.
If I were to use this mechanic In my own game it would be in a fighting/beat em up a style of game. This is because it makes the player memorize moves and relies on the skill and memory of the player rather than just displaying the combo.