What does gazebo::physics::Link::GetRelativePose() return?

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I wrote a model plugin that takes the output of GetRelativePose() of a link and applies a force on said link computed with respect to the output of the GetRelativePose(). I thought the function is supposed to output the pose relative to the parent link, but the force applied to the link seems to be dependant on the link orientation with respect to the world. Not the linear position, but the angular rotation. If the robot faces leftward, the force applied to the link is different than when the robot faces rightward, even though everything else I know of is the same. Any explanation?