Jager mode will automatically initiate when you engage an enemy plane. A ring will appear around your peripheral view to indicate you have locked on and an orange box called the lead box will appear over your target. When this occurs you need to aim your fire at the lead box rather than directly at the plane, this is because the lead box automatically compensates for the direction and speed that you and your target are moving. If you use the joystick while locked on a target it will allow you to fire precisely at the weak points of the enemy plane.