jBPM has a decision node. This is most likely what you need to use: http://docs.jboss.com/jbpm/v3/userguide/jpdl.html#decision.element
You can also look at the testcases (yes, jBPM contains lots of those) to see how it is used.
Indeed, the userdocs, testcases (and sourcecode) are valuable documentation, I've used them a lot already, and managed to get the stuff running after some experimentation. Defining some simple definitions and testcases is a good starting point to experiment.
Step-debug through the code, and you'll learn a lot too.