The lead turn is a "first move" to gain an advantage on your opponent by out-turning him.

As you are merging, begin turning early into the enemy to gain a snap shot, then folloew through with the turn to put yourself on his six o'clock. Ease off the throttle to maintian a tight turn. Keep an eye on the enemy, he should be in your upper canopy view. When you are in the tightest turn possible, apply full throttle while keeping the stick back as far as you can get it without stalling.

Drawbacks:
Begun too soon, the lead yurn can put you in front of the enemy. Also, it is somewhat predictable, easy to counter, and does not present you with an immediate shot.
Timing is key. Do not perform this maneuver unless you are certain your ship can turn with or out-turn your foe's plane, of if his speed is so great that he can not turn tightly.