Under this rule, you replace iterative attacks with flurries. A flurried full attack grants an extra attack every BA +5, but at a cumulative -2 to all attacks. For example, if you have BA +17, you can make up to four attacks at -6 each (see table, below).
|Base Attack||Max. Attacks||Max. Penalty|
When you make a full attack, you can choose how many of your flurries you want to use and thus control how much of a penalty you take. For example, with that BA +17, you could make either a single attack at +17, or two attacks at +15, or three attacks at +13, or four attacks at +11.
Because the math is simpler and the rolls are easier to resolve, you can (and should!) extend full-attacks into the Omega levels, as the table indicates (BA +21 and above).
If you have the ability to flurry as a standard action (e.g., Rapid Shot), then you can still do so, but you cannot combine a standard-action flurry with a full-attack flurry. For example, you have Rapid Shot and BA +15. As a standard action, you can make two ranged attacks at -2 each, but as a full-round action, you can make three attacks at -4 each.
You can combine flurries with two-weapon fighting. The two-weapon penalty is cumulative with the flurry penalty, but the two-weapon-fighting penalty is a single penalty, not one per off-hand attack, and you can still use two-weapon fighting for an attack action.
There are several positive effects and a couple of negative effects of this alternative rule.
The positive effects are simplicity and speed. It’s easier to remember “-4 to all attacks” than it is to do the math to figure out “+12/+7/+2” even though that math is relatively simple. You don’t have to list all of the bonuses/penalties for every additional attack because you know it’s a flat penalty to all of them. Simplifying the math always speeds up combat.
You also no longer have to roll all your iterative attacks separately or use colour-coded dice. You can just roll multiple d20s and check which ones hit because they all have the same bonus and DC.
Finally, this system probably speeds up combat at high levels in particular because when high-level opponents face each other, they have high Defence scores, which means that everyone involved is likely to take single attacks at their highest bonus rather than multiple attacks at lower bonuses.
If there’s no chance of hitting, people will go back to their full-attack option hoping for a natural 20, but that is very easy to resolve: roll a few d20s and see if any of them come up “20.” If they don’t, you’re done.
On the down side, in the aforementioned high-level encounters, you might end up with less damage per round because there are fewer attacks in total and, thus, less total damage. Potentially, you can get more rounds in when they go faster, though, so this effect, could be a blessing in disguise.
The actual math of the switch from iterative to flurries works out in the attacker’s favour—you gain more than you lose—however, you also lose the ability to attack once at your highest bonus and then attack at your lower bonuses, so this system is more “all or nothing” than the iterative system.
As a final note, there are lots of ways to adjust your penalties to hit: Power Attack, Combat Expertise, and Aid Another just to name a few. Under the flurry system, you’ll probably become more comfortable adjusting your full-attack penalty up and down, which means that feats and other mechanics that allow you to adjust your bonuses might become more comfortable, but this mileage is sure to vary from table to table.