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This is a hold position trajectory: A single-waypoint at the current time set to maintain the current joint positions.

When the JointTrajectoryActionController starts, it defaults to holding the current position while it awaits for the first command to arrive.

Note: The code snippet you reference is from the init() method. Something similar also happens in starting(), so it seems that the snippet in init() is unnecessary and IMO could be removed.

Since you are asking many questions about this controller, are you planning on reimplementing it for a custom non-PR2 robot, or are you simply studying its internals?. FYI, a robot-agnostic implementation of this controller should land in the ros_control project sometime in the next 3 months.