My wife & I went for the Harmony test at FMC at 10.5 weeks. As I recall, the first scan is purely a quick viability/dating scan, and the bloods are taken at this first appointment.
One blood sample is sent off to the States by courier (don't believe there is a Harmony testing machine in the UK at the moment). Another blood sample is used to do a more traditional "combined" blood test.
At the second appointment a detailed Nuchal/anomaly scan is performed, and the bloods used to give a risk factor for T21, T18, etc. This will be in the standard format "2300:1 chance of T21", "8400:1 chance of T18", etc.
In addition, the Harmony results are given. Ours were of the format "> 9999:1 chance of T21". Basically this means "all clear". The Harmony test has never missed a case of Downs syndrome, although in extremely rare cases false positives are possible, which then require a CVS to confirm.
The Harmony test works by finding pieces of the Fetal DNA floating in your blood. These pieces come from cells in the placenta, and are perfectly harmless/normal.
If the Harmony test gives you an all-clear you can pretty much forget about the 3 more common trisomies. More and more places are starting to offer the Harmony test now (our local clinic is now offering it), and it won't be long before it is a standard part of the Nuchal process on the NHS. Hopefully the number of women going for unnecessary CVS/amnios will fall dramatically.
Finally, it's worth mentioning that if you go for the first appointment at the FMC in your 10th week, you get the Harmony test free, and just pay £180 for the combined & nuchal tests. This was the deal they were doing a couple of months ago anyway.