If you read on the australian website (IntelliGender® Homepage - IntelliGender?) they have discounts for ultrasounds to confirm their tests' result, and I *think* that if the test was inaccurate then you get your money back.

But basically it tests for male hormone, and the male hormone has a chemical reaction to what every they use and turns green. If your having a girl, then the urine turns orange.
If your test comes up orange, and your having twins or multiples, then they are all girls. If it comes up green then at least one of your twins/multiples is a boy.

90% accurate in LAB testing, I thnk it was something like 83% accurate in the general public. Sexual intercourse can give you a false boy result so best to wait 48 hours after sex before doing a test. PCO and PCOS will give you a boy result, even if your having a girl, and progesterone treatments may also give you a false boy result.