An Action Command (also known as a Timed Hit or a Timed Attack) allows a character to do significantly more damage to an enemy than with a normal hit. Often, Action Commands occur when the player presses the "A" button right before the party member hits his opponent. Action Commands are also used for defense; If the player presses the Guard button at the moment an enemy's attack hits, the character receives less damage.
Super Mario RPG
The Action Commands were originally called "Timed Hits" in Super Mario RPG: Legend of the Seven Stars, and are the key to success in the game. They are used both to increase damage dealt by the party's attacks and decrease damage done to the party by physical attacks. Timed Hits do not actually increase any statistics; they only serve to modify the damage after it is calculated.
There are actually two different levels of Timed Hits; one for timing the command "close" and one for timing it "perfectly". For the party's physical attacks, most Timed Hits are activated by pushing any button right before the attack lands. If an attack is timed close, the damage is increased by 50%, but if it is timed perfectly, the damage is doubled. Any Timed Hit with a physical attack will never miss.
For the party's special attacks, more variation is seen in the commands. Generally, any special attack which requires a single button press will increase damage (or healing) by 25% if timed close or 50% if timed perfectly. Other commands (push a button repeatedly, hold a button, rotate the control pad) increase damage in other ways. For some attacks, good timing will increase the attack's range from one enemy to all of them.
Timed Hits are also used to reduce damage done by certain attacks. Only physical attacks may be blocked in this manner, and even some of those are unblockable. For those attacks which are blockable, a Timed Hit is usually activated by pushing any button right before the enemy attack lands. If the command is timed close, the character will only receive half damage, and if it is timed perfectly, the character will receive no damage at all. For attacks that cause instant death, a normal timed hit will leave the character standing with only 1 HP (and a perfect one will cause no damage).
Paper Mario
In Paper Mario, Twink gives Mario the Lucky Star, a special pendant from Peach that allows the plumber to perform Action Commands. This artifact proves useful in the upcoming battle against a Magikoopa; a fight that occurs after Mario visits Shooting Star Summit for the first time.
Paper Mario: The Thousand Year Door
In Paper Mario: The Thousand-Year Door, Action Commands reappear, and Stylish Moves are introduced. These allow Mario to recharge his Star Power faster. They can be performed from the start of the game, but have no effect until the first time an audience appears to watch Mario's battles. While Action Commands are not used in Super Paper Mario, Stylish Moves do appear.
Super Paper Mario
In Super Paper Mario, Action Commands are not used in battle; rather, they are done when using items. These Action Commands usually implement Wii Remote pointing and shaking.
Mario & Luigi
In the Mario & Luigi series only, Mario and Luigi can entirely avoid getting hit by enemy attacks by using the Action Command. They can jump over their enemies or hit them with hammers to prevent them from attacking. Mario & Luigi: Partners in Time also has a special action command involving the babies in piggy-back mode.