It's real simple to figure. out. Imagine the upper and lower ball joint in your mind from the passengfer's side (front to you're right) of the car, but with them perfectly verticle, making a perfectly verticle steering axis.
Now, imagine moving the upper ball joint in front of the lower ball joint. Think for a moment what this will do. What will the weight of the car do to the tire? Now, since the upper ball joint is in front, the weight of the car now has great influence and will push down, causing the right front tire to swing out and pull to the right.
Now, again imagine the upper ball joint and lets move it behind the lower ball joint. Same side. Now what is the influence of the vehicle weight? Again, the vehicle pushes down. SO, the tire will want to go toward center. It cannot continue in a turn, because of the front end linkage.
THIS is the effect positive (upper ball joint behind lower) has on a vehicle. It is the vehicle weight pushing down on the steering axis that causes steering returnability and road stability. This is also why caster has to be set very closely (under 1/2 degree difference) on each side to keep from having a caster pull to the side with the least amount of caster.
It's not about the spindle being "behind" the ball joints. The spindle hasn't a clue whether it's in front of or behind the ball joints. It's about the tilt of the steering axis. A verticle steering axis has zero caster viewed from the side. Tilt the top of the axis toward the rear of the car and caster is positive. Tilt the top of the axis toward the front and caster is negative.
Negative caster is good for going around turns at a high rate of speed, while, generally speaking, positive caster is good for a street car for good steering returnability and high speed stability.
The next time you see a Mercedes Benz parked somewhere with the wheels turned, notice how it looks like the tire is gonna fall off. Because most Benz cars have a spec of 8 degrees or more of caster.....so when they turn, you can actually see the front of the car rise to whichever side the car is turning to, because the wheel is actually lifting the car UP as it turns, the caster angle is so great. Because not only is the steering axis turning, but it's also driving the spindle DOWN at the same time. Any car with positive caster does the same thing, just not as pronounced as the Benz cars. You can really see the effect of positive caster on those.