When going down the road, the transmission torque converter locks up which causes the transmission to run cooler and increases gas mileage. When you are jockeying your trailer around in your yard, etc. the torque converter never locks up so a lot the energy going into the tranny from your engine is being converted to heat due to the fluid friction in the torque converter.
Think of it this way: the torque converter is a fluid coupling. Envision 2 box fans pointing at each other. When you turn one fan on the other one begins to turn due to the air flow from the first fan. A torque converter works much the same way only it uses oil instead of air. When the torque converter locks up it is like bolting the 2 fan blades together so that there is minimal energy being converted to heat.
Does that help?
2006 23B Hybrid with 10k round bar WDH
2011 F150 4x4 SCREW Ecoboost, Max Tow, Integrated TBC, 3.73 LS axle, Firestone Ride Rite airbags.