There is no difference for processes with or without task.
More than one node can be active at one time. Each "thread" (not in the OS/Java sense) of execution is represented by a Token. Look at the ProcessInstance calls that return tokens, then look at Token.
This is illustrated in the very first piece of code you should have seen, the "Hello World" in section 3.1 of the user guide... so I suspect either I don't understand the problem, or a suggestion to RTFM is reasonable.