Twist and /tf listener question

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I'm making a /tf listener that listens the output of a kinnect device.
I'd like to publish the velocity of the /left_hand_1 child of the /tf. So far, looking at the Transformer::lookupTwist function, I have these bits figured out but am missing pieces:

Transformer::lookupTwist("/left_hand_1", "/torso", "reference_frame", "reference_point", "reference_point_frame", ros::Time::now()+ros::Duration(0.1), "ros::Duration()", vel_msg);

Format of the Transformer::lookupTwist can be found here. My question is about the "reference_frame", "reference_point", "reference_point_frame", parts of the function... what do they refer to? Is the "reference_frame" the same as how I would observe the "/tf" values in rviz? ie, "openni_camera"...? What about the "reference_point" and reference_point_frame"?

Thank you for the help.