The Master Command and Support Materia glitch in Final Fantasy VII involves pairing the Master Command Materia with a Support Materia, which will apply its effect to most commands the player currently has equipped, including Attack, Slash-All, Flash, 2x-Cut, 4x-Cut, Item and W-Item, and Throw.
Limit, Magic, Summon, E.Skill, W-Magic, and W-Sum. are unaffected. This means that magic and summons can only benefit from Support Materia by linking them directly to the Magic or Summon Materia, or to Master Magic or Master Summon. Enemy Skills never benefit from support effects.
Example: By equipping a Master Command Materia with the Support Materia, and then having Double Cut or Slash-All equipped somewhere else on the character, whenever the player activates the command from one of the two said Materia, whatever Support Materia is linked to the Master Command will be used with those commands. By using this glitch a player can attack five times using 4x-Cut from the Double Cut Materia (by linking Added Cut with Master Command), or attack using Slash-All and gain HP back (by pairing Master Command with HP Absorb).
Since this also affects the Item command, Added Cut paired with Master Command means the player will attack after using an item. If a revive or restorative item is used on a party member with this setup, the player will attack the character immediately after using an item on them. HP and MP Absorb also have the opposite effect if used with an Item command and a healing item; gaining HP loses 10% of the effect gained.