The angle is you are referring to is called "reactive caster" (or sometimes called active or dynamic caster). It provides dynamically changing kingpin angles depending on how much the suspension is compressed. Reactive caster is controlled by the angle of the caster blocks. The more angle, the more the kingpin will stand upright when the suspension is compressed. With reactive caster, you'll have more positive caster in the straights and less in the turns. Positive caster tends to straighten the wheel when the vehicle is traveling forward which enhances straight-line stability. Bottom line is, your car will be just as aggressive in the corners, but easier to drive in the straights. It doesn't really change how loose/tight the car is, although it could technically make the car feel looser in the corners because the kingpin stands upright giving you more agressive steering (but not any looser than what you have now). We've done a lot of testing with the different blocks, and found that the lap times increased and were most consistent when running two 10 degree blocks.
You should note that a 10 degree caster block does not give you 10 degrees of caster, it gives you 10 degrees of "action" so to speak. Spacers are used to set the initial amount of caster, and the reactive caster block changes the kingpin angle under compression.
I see a lot of people run it different ways. Many times I see 5* on the LF and 0* on the RF. I have also seen 5* and 5* and 10* and 10*.
Like James said, the idea is to give the car more stability when straight, yet still be just as aggressive when turning. Ideally, you would want to try several different combinations of it to see where you like it and your car likes it.
Think of it this way, without reactive caster, you have to choose a static caster (kingpin angle) that is with the car at all times. Because 0 degrees of static caster is often too squirly down the straights, people tend to add 2 degrees (or so). You'll have 2 degrees in the corners and straights.
But with reactive, it varies on the straights and turns. Therefore, you can set it so that it starts a 2 degrees and ends up at 0 degrees in the corners. Yes, less caster makes the front end more agressive. So you get the best of both worlds: agressive in the corners and stable in the straights.
We have a video on youtube about the changes in a reactive front end systems.
Check it out
Now remember this is the movement in a full .200 coil bind situation. If you run you car just add some graphite dust to the king pins set the car down get some laps in without crashing or pushing down on the suspension. Check how much of the powder has been cleaned off from the pivot balls and that is the amount of travel you are using while racing with that suspesion set up. Every suspension set up is different so i cant tell you how much it is moving. But you cn then give your self a simple formula based off the total movement in this video with a BM front end to figure out the total reactive caster and camber you are achieving.
PAN Cars take to the smallest of changes so even if you do the math and you are only seeing a .0002 of a degree of caster change in the corner it can make the difference on the track.