A kite flies and generates power because it acts as a wing. Air moving over the top of the kite (wing) travels faster than air moving below it. This faster moving air travels upwards due to the curvature of the wing, which creates less pressure above the wind, which generates lift.
When the front lines are under tension via their attachment to the chicken loop and the rear lines are loose (bar pushed out):
The "wing" is flattened - wind speed below and above the kite is similar.
The angle of attack is reduced
Very little lift and power is generated
In this situation, the kite is depowered.
When the rear lines are under tension (bar pulled in) and the front lines are loose:
The "wing" is curved - wind speed above the kite travels up and is faster that the wind speed below it.
The angle of attack is increased
Lift and power is generated
In this situation, the kite is powered.
Learn to "throw the bar" to depower the kite if you get dangerously overpowered. Hanging on the bar in this circumstance could get you lofted or dragged into an obstacle.