This test is given a day or two after your last exam of 2nd term. It tests your retention up to that point with questions that are both external to the university (USMLE type) and internal (you've probably seen these questions on previous tests). It's a 200 question brute over 4 hours. Everyone feels like blowing this test off because a) failing it doesn't stop you from doing anything and b) passing it doesn't help you do anything.
I disagree. Your score on this exam ranks you against your class. You'll get a letter in the mail telling you your rank, your Z score, the mean and a breakdown of how you did in every section on external and internal questions. If you're serious about doing well on the USMLE, I don't know why you wouldn't take advantage of this test. Normally you have to pay Kaplan to tell you your weaknesses; the school is offering it for free. The school maintains that your rank is kept in house and does not make it onto your transcript. I haven't graduated yet so can't verify this. Students maintain that part of the decision-making when it comes to your hospital placement in 3rd and 4th year has to do with your rank, everything else being equal.
Moral of the story: doing well can't possibly hurt you, and doing poorly can't possibly help you. I for one was open to the possibilities, so tried to do my best. The best way you could possibly prepare (for the BSCE or USMLE) is to teach it. If you pulled a B or better in Anatomy, Biochem, Histo or Embryo then tutor the class. It takes two hours out of your week every week and it's a great review that your classmates aren't getting. And don't let fourth term scare you too much: my roommate and I team-taught Physio and Neuro and it was fine. We even had fun doing it.
So good luck and enjoy your summer.