Final answer:
To lower a fully loaded trailer's landing gear, turn the crank to extend the legs until they support the trailer's weight. A bouncing trailer on the highway is likely nearly empty, as a heavy load would stabilize it and reduce the amount of bounce.
Step-by-step explanation:
To lower the landing gear on a fully loaded trailer, you have to engage the handle attached to the gear mechanism. This handle, often called a crank, will be turned in the appropriate direction to extend the landing legs to the ground. As you crank, it is important to do so evenly and steadily to avoid tipping or damaging the trailer. Once the landing gear is fully extended and the weight of the trailer rests on the landing legs, you can then unhitch the trailer from the tractor.
Regarding the observation on the highway, if you see a trailer bouncing up and down slowly, it's more likely that the trailer is nearly empty. A heavily loaded trailer would have less bounce because the weight would create a stabilizing effect, meaning less oscillation when the trailer encounters bumps or uneven road surfaces.