In another post by Square Enix they have explained how battle regimens work and the effects they have. This is a game mechanic that isn't very well understood by the FFXIV community so the information given by Square Enix will hopefully get more people using them.

As taken from the Lodestone site;

"Battle Regimens are combat tactics which party members may employ against a single enemy. By coordinating and executing battle commands in a precise order, players may cause certain additional beneficial effects to occur. A Battle Regimen begins with the stacking of battle commands. All party members who wish to take part in the Battle Regimen must stack their actions in the correct order against the desired target. One of the members then initiates the Battle Regimen, and the stacked actions will be executed automatically.

Executing a Battle Regimen
1. Consult with other party members to determine the best Battle Regimen to use against a specific target. The battle commands and the order in which they are to be executed should all be decided.

2. Wait until your position in the queue comes around to switch to Battle Regimen mode and select the battle command you wish to stack.
*A single party member cannot stack more than one action in a Battle Regimen.

3. After all participating party members have stacked their battle commands, one of the party members selects to initiate the Battle Regimen.

4. Once the Battle Regimen is initiated, the stacked battle commands will all be executed automatically, chaining together to create additional effects.
* Please note that factors such as distance from the enemy may prevent stacked battle commands from being executed once the Battle Regimen is initiated. In these cases, the Battle Regimen will not end, but will instead proceed on to the next command in the queue."

The list of additional effects can be found below.

Additional Effect	Combination	Example
Lowers a target’s physical defense	Normal attack => Normal attack	Light Slash => Heavy Strike
Lowers a target’s magic defense	Normal attack => Magic attack	Light Slash => Phantom Dart
Increases a target’s casting time/raises MP costs	Normal attack => Magic spell	Light Slash => Fire
Lowers a target’s TP progression/raises TP costs	Normal attack => Weaponskill	Light Slash => Red Lotus
Maintains the effectiveness of class abilities when used by different classes	Magic spell => Magic spell (both from same class) Weaponskill => Weaponskill (both from same class)	Trunksplitter => Brandish
Increases player damage	Weaponskill => Weaponskill ? Magic spell (weaponskills from different classes, finishing spell is optional) Magic spell => Magic spell => Weaponskill (spells from different classes, finishing weaponskill is optional)	Blizzard => Scourge => Puncture

The next part of the article talks about incapacitation. This basically means that by using certain weapon skills on the right kind of monsters you can do damage to parts of the monster. This serves three purposes, to weaken the enemy, to prevent it from using its special attacks and to influence the loot that drops. So it is definitely worth using these. But how do you know which skill to use and on which kind of monster? The tables below will help you.

The first table shows the kind of weapon skill that affects the body part and which part.

	Head	Limbs (Right)	Limbs (Left)	Limbs (Lower)	Other (Right)	Other (Left)	Other (Outer)
Sword	--	Riot Blade	--	Red Lotus II	Spinstroke II	--	Luminous Spire
Hand-to-Hand	Concussive Blow II	--	--	Seismic Shock II	Victimize II	Simian Thrash	Pounce
Axe	Skull Sunder II	--	Maim	--	--	Brutal Swing II	Trunksplitter II
Polearm	--	Moonrise II	Chaos Thrust	Skewer II	--	Twisting Vice	--
Archery	Shrieker	Barrage	Foeseeker	--	Bloodletter	--	--

The second table shows which body part can be damaged for each genus of monster.

	Head	Limbs (Right)	Limbs (Left)	Limbs (Lower)	Other (Right)	Other (Left)	Other (Outer)
Hippogryph	Head	--	--	--	--	--	--
Opo-opo	Head	--	--	Legs	--	--	--
Gnat	Head	--	--	--	--	--	--
Cactuar	--	--	--	Legs	--	--	--
Giant crab	--	--	--	--	--	--	Shell
Salamander	--	--	--	--	Right Wart Cluster	Left Wart Cluster	--
Wolf	Head	--	--	--	--	--	--
Boar	Head	--	--	--	--	--	--
Ahriman	Eye	--	--	--	--	--	Tail
Wight	Skull	Right Humerus	Left Humerus	Femur	--	--	--
Dodo	Head	--	--	Legs	--	--	--
Coblyn	--	--	--	--	--	--	Ore Cluster
Aldgoat	Horns	--	--	--	--	--	--
Imp	Head	--	--	--	--	--	--
Flytrap	Maw	--	--	--	--	--	--
Antling	Mandibles	--	--	--	--	--	--
Humanoids	Head	Right Arm	Left Arm	Legs	--	--	--

Source: The Lodestone