the idea is, if you kick ass in internals, but, say, have you dog gets hit by a car, get a cold, have really bad period pains, something that disadvantages you, for whatever reason, the day of your exam, you will still be aligned up via your internals.
similarly if you slack off on your internals, itll drag your marks down.
the school sends in your assessment marks, and thats 50% of your hsc mark, or something.
its been explained to be a million times but i cant never restate it properly, or forget something improtant, but thats the gist i think.