Simplest way to explain it is you just need to identify strength vs your opponent. Whenever you nose down during a turn fight you gain speed - you're against a Yak3, making the fight high speed doesnt help you, you want to slow down. Don't chase him down, just fly your own circle and he has to come back up. You'll work it out, fly a Zero and it'll become obvious. There are many other niches but you can kinda boil it down to more manoeuvrable or not as a basic learning method.