You have 4 way thrusters - up, down, left, right.
If you are looping in a circle, going up - pushing the up thruster while going up will push your ship "up" as you go up, giving you a slightly steeper angle than you can manage without it - often getting a shot before the other guy can turn if they are maneuvering better than you are.
That's the easiest way I can describe it. The same works if you are looping left to use the thruster to move to the left.... So on and so forth. As you get a feel for using multiple powered direction controls, you will do a bit fancier stuff to better control the fight.