Here's a good explanation - http://pistol-training.com/archives/1361
Some manufacturers have a 6 o'clock hold and other have a hold over (or combat hold, maybe?) Anyway, with almost all iron pistol sights, you line up the top of the front sight with the top of the rear sight. The dots are there to aid in acquiring the sights in low light or for faster follow-up shots and transitions along with it being faster to get your sight picture.
Now, for the Straight 8's, I have them on one of my Nighthawks and my VBOB. I much prefer them at speed over the 3 dot configuration and prefer a single dot front over both. It all depends on how we shoot here. For me, I shoot fast from the draw and fire a few rounds as fast as possible then transition to another target, fast. So, a plain or single dot rear sight draws my attention to the front sight considerably faster than a 3 dot configuration. Also, I prefer a U notch about .150 with a .125 front for speed. Of course, the fiber optic with a plain rear is extremely fast as well. You have to figure that most of the top shooters that are fast use plain rears with a plain or fiber optic front. Not all, of course, but most. Personally, I still have trouble with all black sights.
If I'm shooting slow and for accuracy, I'd prefer all black or a single dot configuration with a .125 rear squared notch and a .125 front sight. Three dots are fine here too, but you have to remember that you're lining up the tops of the sights (whether a 6 o'clock hold or holding over the target) and not the dots. I have a few guns with 3 dot Novak's that do not line up when the tops are even.
I just put some Warren Tactical 2 dots on my M&P9 and TruGlo's TFO front on my Gen 4 17 with their 2 dot on the rear due to not being able to find a plain rear that works with their front. I just wanted to try them since two of my Sigs have their TFO (tritium and fiber optic) front with tritium rears. I'm changing that out to a plain 10-8 .140 rear with a Warren tritium .125 front. My 1911's all have Novak's or Heinie's, but soon they'll all have 10-8 plain rears with tritium front, except for my Nighthawk PDP Commander, which will have the Heinie QWIK .154 rear with a single dot and a tritium front with a white outline.
Now, for just about anyone out there who TRAINS AND BECOMES PROFICIENT with both the three dot and two dot set up, will most like find that the two dot or single dot with a wider rear U notch are faster for follow up shots and target acquisition. There is nothing on the rear sight to slightly distract your vision. A small single yellow dot or a plain rear will allow the front sight to be much more obvious. A U notched rear will also allow this....it has something to do with the only 90º angles being at the tops or and the front sight.
Iif you shoot a three dot setup for years then just give a two dot set up a 2 mag try, then of course the three dot will be faster for you. You can become proficient with just about anything, but sometimes certain things just allow you to do it quicker and to a higher degree. The only way to know is to give it a fair chance, and by that I mean shoot it as your primary sights for an extended period of time like 6 months or more. THEN, and only then, will you know they are definitely not faster. I hated them at first, then forced myself to use them. Once I understood their purpose (SPEED, and you may slightly sacrifice accuracy) and gave them a fair chance, I saw m,y times improve. I shoot three different types of sights every couple days. Soon, they'll all be two or single dot sights, no more three dot sights.
* At night, you need to remember there's a good gap between the two tritium dots. If you don't train regularly with them, it's easy to try to get them as close to each other as possible. I've done it many times!
Keep in mind that the wider the rear and if it's a U notch, this will also aid in speed. These things together are very fast. Even if so-and-so is faster with three dot sights, that doesn't change the fact that single and two dot sights, especially in a wide U notch are just plain faster. I was faster with three dot sights when I first tried two dot sights. Then I understood that I was to completely leave the rear dot out of the sight picture (unless it was pitch black out) and I became MUCH faster. It's very hard to do that, for me, with the three dot sights.