The rules incompassing Action Points is to mitagate the constant use of powerful feats such as Whirlwind Attack and Great Cleave from being 'spammed'. By definition, a Feat is suppose to be something of unusual or especially incredible ability. Further, if warriors can consistently use their feats in stacked combinations without limit, why bother with a spellcaster other than for flavor? To balance this issue, we have placed a rule to govern the use of the combat abilities that, for all intents and purposes, are the melee class's version of spells.
First is the matter of calculating your appropriate amount of Action Points. This is done rather simply. At 1st Level, you look to your Primay Physical stat. This will be the highest score between Strength, Dexterity, and Constitution. This is to reflect your character's combat style as generally the score you placed highest will be your character's emphasis in battle. In the event that more than one stat is the same, pick the one you feel you are more likely to add points to or that better reflects your mental image of the character's focus. [For example, a character focused in throwing weapons would require both a good Strength score for damage and a good Dexterity score for to-hit. In this instance, you are likely to have similar stats in both scores. In this instance however, your character is focusing on ranged combat so Dexterity is the proper choice.] Once this is established, you take that attributes base modifier plus 1 to figure your character's starting Action Point pool. [In following the previous example, say your characters Dexterity score is 16. This gives you a base modifier of +3. So at Level 1 your character's Action Point pool would be 3+1=4]. As you level up, your character will continue to gain more action points every level. Each time you level up your character will gain a number of Action Points equal to your base modifier. [The same character from before would then calculate his Level 1 point total of 4 and add his chosen base modifier again at level 2. 4+3=7]
If you should increase your chosen modifier, the points you have already accumulated are not adjusted - they are not retroactive like changes to Hit Points due to increased Constitution. Further, do not calulate bonuses to your modifiers from items when calculating your Action Points.
How Much do Feats Cost to Use?
Firstly, not all Feats cost Action Points to use. In fact, a great many of them cost absolutely nothing as they are completely passive-related bonuses. For example, take the Feat Weapon Focus. Your character has spent the required time training with and using this weapon already to gain this bonus. The nature of this bonus is that it gives you a static +1 to your To-Hit with that weapon. When asking yourself if a Feat should cost Action Points, the basic question to ask yourself is, "Does this ability require me to conciously make an action for it work or is it somthing my character knows or has innately?" Another good example for comparasion would be Combat Reflexes. This ability refers to the fact that your character is either naturally or from intense action/training very quick to respond to threats in combat. Therefore, as it gives you a static effect, Combat Reflexes is considered a Passive effect. A basic example of an activated ability would be Power Attack. Your character is conciously making the effort to put more force into their attack making it cost Action Points. Some other examples of Passive Feats would be Great Fortitude, Weapon Specialization, Improved Trip, Wind Stance, and Spring Attack. Some other examples of Feats with cost would be Whirlwind Attack, Acrane Strike, Combat Expertise, Strangler, and In Harm's Way.
Feats have varying costs for usage based on their 'ranking'. The base cost for using a Feat that requires 'activation' is always 1 point. In essences, the 'ranking' of the Feat is based on the number of Required Feats to use it. Once this is established, you will then know the cost to use that Feat. For example, say your character has the ability to use Great Cleave. In order to have Great Cleave, you must first already have Power Attack and Cleave. Since the base cost for any given Feat is 1 and Great Cleave requires you to have 2 other Feats, the cost to activate Great Cleave is 2+1=3. A Player must declare the choice to use a Feat prior to its activation and then spend the points necessary to use it. Even if the player misses the attack, they lose the Action Points [just like a spell]. Also, you must spend the necessary points each time you choose to use the Feat. However, you only need to spend the points once a given turn. So if you activate Power Attack on your turn, the effect remains on each attack made that turn until the following turn when it would need to be activated again.
You MAY use multiple Feats in a turn, however you must pay points for each one used individually. As an example, a player may decide to use Power Attack and Great Cleave in the same turn. The player first spends the points for the lowest Feat used [in this case Power Attack with a cost of 1 as it has no pre-requisite Feats] and then on up the scale which in this case was Great Cleave [which has a cost of 3]. Therefore, your total expended points would be 4 for that turn.