When I told a friend of mine, he also asked me if I had to take a test. What does this entail exactly?
You have to answer 200 questions (they call it the "Oxford Capacity Analysis"). Here's a sample (from Wikipedia):
3. Do you browse through railway timetables, directories or dictionaries just for pleasure?
6. Do you get occasional twitches of your muscles, when there is no logical reason for it?
27. Do you often sing or whistle just for the fun of it?
30. Do you enjoy telling people the latest scandal about your associates?
59. Do you consider the modern prisons without bars system "doomed to failure"?
69. Does emotional music have quite an effect on you?
105. Do you rarely suspect the actions of others?
124. Do you often make tactless blunders?
196. Do you sometimes feel that your age is against you (too young or too old)?
While answering, you have to hold metal cans connected to an "e-meter", which, as has already been said, is just a wheatstone bridge. If the gauges move when you answer, it's a sign that something is wrong with you. Protip: hold the cans as lightly as you can, and don't squeeze if you want to get good results. The slightest squeeze will give a bad result.
Good or bad results, the test is set up in such a way that no matter what you answer, they'll find something negative, something "wrong" with you. They then say that they can help you with those exact problems, and try to set up an appointment in one of their places. They also try to sell you books before letting you go.
The daughter of a Norwegian politician killed herself a few years ago after taking one of these tests on the street. They found so much "wrong" with her that she became suicidally depressed. So don't try them, even not just for fun, if you don't know that their results are actually meaningless.