If u have a jet in front of your "face" than u hold the aimkey. As long as u press the aimkey, your jet follows the enemy.
If u loose the aimtarget, u must fly "alone" until u spot on an enemy again. What u have do do when u stick on an enemy : shoooot !
For the jethelper u have to activate it with the del button. A message will appear like "jethelper enabled".
David