If you go a bit deeper you will find that there is a bidirectional reletionship created between super and subprocess.
subProcessInstance = (ExecutionImpl) processDefinition.createProcessInstance(key);
next go into that method:
if (superProcessExecution!=null) {
// establish the bidirectional relation between super process activity instance
// and sub process instance
ExecutionImpl superProcessExecutionImpl = (ExecutionImpl) superProcessExecution;
processInstance.setSuperProcessExecution(superProcessExecutionImpl);
superProcessExecutionImpl.setSubProcessInstance(processInstance);
}