Compression = dampens the rate at which the forks compress, more dampening helps prevent you blowing through all the travel easily, less dampening makes the fork feel plusher but you will dive more under braking and when you point downhill as you pointed out above.
Rebound = slows the rate which they extend, too fast and they feel like a pogo stick ready to spring you off, too slow and the forks will get shorter and compress down with quick sucsession bumps as they have no time to recover.
Positive air = basically just your spring strength, heavyer rider = more air. inflate to a pressure that gives the correct sag.
Negative air = this helps combat sticktion in the initial travel of the fork by effectivly changing the spring rate through the first inch or so of travel. The more negative air, the easyer the fork will compress at the begining of the stroke, less air and the force required to initiate travel will be slightly higher giving a stiffer fork. The trade off to having plusher forks with slightly more -ve pressure is that you will go through the first part of the forks travel easyer and dive more than if you have less negative air.
If you have a non poplock compression adjuster you can add a little compression dampening to slow the dive without locking the fork out. However, if your stuck with lockout on or off with a pushlock/poplock, you can lockout the fork and fettle the blowoff pressure to get a similar effect.