The amount of detail is in accordance with what PTS are being used. For a 135 check, if the requirement for the pilot is to just be a commercial pilot, the check is to be at that level. If the operation requires an ATP certificated pilot, then the check is performed at that level and according to the appropriate PTS.

With that said, ridiculous drawings and questions about what way the flyweights move can only serve to confuse and it becomes somewhat of an ego-trip for the checking examiner in a "well guess what I know that you don't"?! situation. An examiner that asks you to draw something right off the bat without asking you to explain it is a red flag and sign of a poor examiner.

What is important? Well, what happens when you lose oil pressure?, and from that you should be able to piece together what happens when the prop goes towards high RPM/low AoA or low RPM/high AoA, or what happens if you increase RPM or decrease. You can understand these things just fine, perhaps using a diagram, cut-aways and other resources in the LEARNING phase, but the checking phase is to make sure the applicant can meet the standards, not draw a diagram, and the pompous attitude of those who do expect diagrams to be drawn needs to stop. I'd say a good 98-99% of the time, the EXAMINERs "understanding" of what the diagram should look like or how it operates is wrong, as it's often been simplified to allow for the concepts to be understood, but it often doesn't "technically" work as drawn or there are often missing parts. How many times does the examiner understand that all electrical parts ground to the plane, the negative terminals ground to the plane, electrons flow from negative to positive and therefore flow "backwards" into the airframe, then back into the components, then into the positive leads to the battery/alternator/generator? That it makes a circuit is an important concept, but if an examiner doesn't really know what direction current flows, they shouldn't be asking people to show or draw the BS "journey of an electron", as is commonly done on checkrides. They should be asking situation questions about the electrical system, failures, what is affected, what might happen in various scenarios, etc.

I'm not implying that all one needs to know is "what happens when you lose oil pressure". I am stating though that the instructor should focus on ensuring the concept is understood, using whatever means is necessary, and the examiner should not be playing an ego-trip during the checkride and expecting the applicant to far-exceed the standards and somehow match up to their "greatness".