The SAT essay is the newest portion of the SAT, and was introduced during an overhaul of both the test and the scoring system in the early 2000s. The essay is the very first section of the SAT. The student is given the prompt at the beginning of the test, and is then given 25 minutes to write an essay about the prompt. The essay must be written in pencil, and it is then scanned into a computer while the rest of the test is processed by a ScanTron machine variant.
For obvious reasons, the essay cannot be scored by computer. The first, and biggest obstacle, is handwriting. It takes more time and energy for a human to convert the (many) essays a computer could not read into readable script than for a human to simply read and grade them all. The second is that no mechanism exists for evaluating the quality of an essay by computer. Although such tools as the Flesch Kincaid reading ease measure certain characteristics of an essay, it is much too easy to use gimmicks to get around these. (The Flesch Kincaid system relies on word length, so simply using big words, even when they are inappropriate, would be easy.)
After being scanned into a computer, each essay is sent to two human graders for analysis. Usually the graders are English teachers at the high school or middle school level, and they are given only vague criteria. There are only three scenarios in which a student receives an automatic “0” for the essay. There zeros are reserved for when the essay is illegible, even to a human, when the essay is unrelated to the prompt (the prompts are very broad, to allow for many responses, but some people can’t even stay within that), and when no essay has been written.
Assuming none of those are the case, the grader is given a 1-6 scale, which basically says that the “excellent” essays get a 6, slightly worse than excellent gets a 5, and so on. Of course, there is always the possibility of an unfair or uncaring grader. The Collegeboard safeguards the system against this by sending each essay to two graders, and having it graded by a third grader if the first two grades differ by more than one point. Assuming the essay is not graded a third time, the two scores are added to one another to yield a score out of 12, and that score is factored into the Writing section score.