How do I know if the distrubutor on my 1991 S-10 Blazer is positioned exactly right?

When I turn the distributor counterclockwise the engines idle goes way up. When I turn the distributor back the other way the idle goes down with every little bit that i turn it. why does it idle so high when I turn it the one way? And when I turn it clockwise, why does the idle go down? Is the lower idle a good thing when adjusting the distibutor? Maybe if someone could explain how turning the distributor works, it would help me out. Please give me anything that might help. Thanks.