GCC C++11 Condition Variable Wait Internals

I'm hunting down a bug we have with some messy thread/condition variable classes being updated to use C++11 threads. During the course of the hunt, I've come across the following in the GCC codebase:

  template<typename _Lock>  void  wait(_Lock& __lock)  {    unique_lock<mutex> __my_lock(_M_mutex);    _Unlock<_Lock> __unlock(__lock);    // _M_mutex must be unlocked before re-locking __lock so move    // ownership of _M_mutex lock to an object with shorter lifetime.    unique_lock<mutex> __my_lock2(std::move(__my_lock));    _M_cond.wait(__my_lock2);  }

Despite the comment, I'm having difficulty understanding the purpose of the move constructor here to __my_lock2. Why is __my_lock moved to __my_lock2 here?