Tweetsimple physics.
throw a ball at a wall with 2 pounds of energy it then stops.
2 to 0 is a change of 2 pounds
throw a ball at the wall and it bounces back to you. thats now 2 pounds of enenrgy to the right down to zero the bounces back to the left at 2 pounds of energy... 2+2 = a change of 4 pounds for that split second that it takes to change directions.