Rule 42 deals with propulsion.
Most judges have been involved more or less in on-the-water judging of rule 42. In order to get a consistent level ISAF publishes Interpretations of rule 42. Some say the interpretations are very difficult, but like in most things, it is a matter of how much you study them.
The latest help to understand these interpretations appeared on the ISAF web site yesterday:
A brand new Quiz about rule 42: RULE 42 QUIZ
The object of the exercise is to first make the quiz and then have a look at the RULE 42 QUIZ ANSWERS
And no cheating.....