Fugitive is dead on with his comments on the merge.
I'll add a little spin of my own. So many sticks go for the HO, even some of the more experienced ones let their temper/aggressiveness get the better of their judgement at times and they fly
pipper-on/HO on the enemy. I like to appear like I'm one of these; normally my opponent accepts the "game of chicken" and fly's right at me. When I see that they've committed to this I fly at them until I get the angle and distance that looks best and then roll/pull first. Often it is the
"underneath them" merge because I know I'll pull up first as they were still trying to get a shot in but, it doesn't have to be that way. If I can tell I have more energy I'll do my
"pull up in front of them" merge and come down on top. It's like setting up a quick rope and at this point the fights over and he is pooping in his pants. You have to pull up earlier in the merge and regardless of how you do it you have to be mindful of what plane is against you. Doing any of the "game of chicken merges" is a no-no against cannon armed planes.
Others in the post have talked about managing separation and this is a good way to get that separation started. Like a bullfighter sidestepping a bull.
I prefer to fly the F4U and because it rolls well for a quick direction change; these tactics get results. If you have misjudged your enemy in the
"pull up merge" it's easy to quickly roll inverted and pull down a little then level off. At that point you're moving in the opposite direction from the enemy at high speed and they never got a shot at you. You can go to friends, go home, go to the bar....... whatever.
I hope this is useful.