It's a pity - but really this method is not implemented.
I advice you to use something next :
StartWorkflowInstruction instruction = new StartWorkflowInstruction(session);
ProcessInstance result = bundle.ContextRepository.StartWorkflow(instruction);
Note, that in StartWorkFlowInstruction you can specify additional information (such as ProcessDefinition).
If StartWorkflowInstruction has one subject - Tridion tries to resolve it as Bundle (and in this case it will start workflow for all bundle items).
Also with help of this method you can start workflow for specific list of subjects.