I managed to score a 14 on the PS. I don't know "how" I did it exactly, or if I could explain a method to do that well. It's like asking da Vinci how he painted the Mona Lisa -- he could talk to you about brush strokes and perspective, but ultimately he's just got this "thing" that some people have and some people don't. If there was a way to get a 14 that was easy to explain and convey, everyone would be getting the same score because they'd all be doing the same thing.
I didn't take a prep course -- I registered as late as humanly possible, and was busy working all summer (I took it in August). I agree that one should not, in general, have gaps in their knowledge. When I took the MCAT though, I had not yet taken the 3rd quarter of physics (at my school they were Winter, Spring, Fall). There was a whole passage on circuits and resistors -- I had never seen them before so I had to guess. And yet I walked away with a 14. How?
For starters, most people answer a ton of questions wrong on the PS section -- so don't sweat it when you don't know an answer -- just make an intelligent guess. Also one must remain relaxed and loose -- stress will kill you on this test.
If you have taken gen chem and physics AND DONE WELL, then you will probably do well on the PS section. I found that I did not really need to remember every last equation. The basic equations and concepts will carry you through this test. Also, don't make the questions more complicated in your mind than they are on paper. When you read a question, don't just read the question, attempt to work out the answer, and then look at the choices to see if your answer matches an available choice. Read the question AND the choices, and then work out the answer. This may save you valuable time, since there are a bunch of questions with very obvious answers. I finished the PS section 20 minutes early, but many, many people don't finish. Thus, I must admonish you all to not waste time -- the key to MCAT test taking is to quickly get to the essence of each question -- don't waste time thinking about stuff that does not DIRECTLY relate to the answering of the question.