Ok, I don't know the absolute best answers for those questions, but I can offer you a good overview so that you understand them better.
1: BnZ = Boom and Zoom. This is a type of attack on an enemy plane that is useful if you are flying a plane that has high dive speeds and very good control at those high speeds. Typically, for a BnZ attack, you will start at a higher altitude than your target. You will dive in on the bandit to make a high speed firing pass, trying to hit your target before he can react. As you pass you immediately pull up into a climb and use your speed to regain the altitude that you lost in the diving attack. This allows you to get back above the bandit and control the fight by deciding if you want to go back down and re-engage him or not. I don't know if that is clear, but I believe it is fairly accurate.
2: Compression. This amounts a loss of controlability of some planes in a hgihg speed dive. As the plane increases speed and approaches the speed of sound, shockwaves form that disrupt the airflow over the control surfaces, particularly the elevator. This makes it very difficult for a plane to pull out of a very high speed dive. A lot of times, the lower you get, the more you regain control, but this is not always the case, as sometimes the plane is going too fast to make an effective pull-out. How this affects you in Aces High is that as you increase your speed in certain planes during a dive, you will lose the ability to pull out of a steep dive. This leads to your planes digging a deep hole into the ground. Planes that are notorious in AH for doing this are the P-38, Bf 109 and P-47. The Me 262 is also very prone to this due to its high speed.
One other thing about compression, throttling back may help sometimes, others it won't. It depends on the plane and how fast you are going. Also, sometimes, you can use elevator trim to pull the nose out of the dive.
Hope this helps. You might also look at
SimHQ in their Air Combat Corner. There are some very excellent write-ups concerning air combat and maneuvers that will help explain these questions and any others you have. As always, feel free to ask more questions as they come up.
-math
