How does the PR2 IK plugin work to solve the redundancy

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Hi guys!

I see the script of PR2 IK plugin. At first, I thought PR2 just locks one joint and makes itself a 6-DOF robot to solve the IK. In this way, the locked joint should be always the same value as the initial value. But when I output the value of the locked joint by moving the end-effector in MoveIt!, I found it changed in some pose. I don't find the script where to change the value of the locked joint, but it does happen. I wonder how the value changes and call which function to change the value of the locked joint!

I will be grateful if anyone could help! thanks a lot!