The min takeoff weights on the b737-700igw for different engine de-rates (the heavier you are the more engine power you're allowed) relate to amount of rudder force available during a takeoff with an engine failure. The short body and the big engines mean that you're at risk of losing directional control both on the ground and in flight if you exceed the allowed de-rate. So don't push that thrust levers past the green caret unless you're above your flap maneuver speed.