In the world of language instruction and testing, “proficiency” means the ability to do things with language. In a foreign-language course, the student moves from the easiest kind of tasks (such as introducing yourself) up the most complex (such as arguing for a point of view on a social issue). In a test, the examinee is invited to tackle a series of tasks, and proficiency is rated according to how successfully the tasks are handled.
Each test is unique, and the testers will not have a set list of tasks made up in advance. In large part, the tasks will be tailored to you as an individual, considering your experiences and interests. To get an idea (a sampling, not a full list) of the kinds of tasks that might be offered, have a look at this link.
You will see that the simplest tasks are on the list as S1, and tasks get more complicated going up the scale. In general, the test will start at the easiest level, and more up as you show the ability to handle more complicated things. Sometimes the testers will move down the scale a bit and ask you to do something easier. This is meant as a sort of break for you, in between more challenging tasks.
Many students will be familiar with tests that are all about grammar. Those are not proficiency tests! In a proficiency test, grammar is important only in so far as it impacts the ability to perform the task: Did you ask the question in a way a native speaker can understand? Does the story you told make sense, or is there confusion about what happened when? Does your point of view come across clearly?
Here then are a few tips on how to do well in a test of speaking proficiency.
Try to relax and be communicative. Don’t hold back because you are worried about making a mistake.
Say as much as you can. Don’t try to find the shortest answer you can give and then stop. Keep talking, give more than you were asked.
Don’t worry about telling the truth. This is a language test, so it is perfectly OK to adjust the facts to fit what you are able to say. If you can’t think of a way to say “I sprained my ankle,” you could try “I broke my leg.” It will probably fit just as well into the story you are telling.
In the same way, don’t worry if any facts you mention are possibly wrong. If you say Pierre is in North Dakota you will not lose points on your test.
Especially for higher-level tests, you should be generally familiar with current events. If the testers ask about one topic and you say you don’t know much about it, that’s fine. If they can’t find a topic you do know something about, it may seem that you are trying to avoid speaking about such things.
There is no way to cram for a proficiency test, and a short bit of practice can only help so much. If you are very rusty, it might be a good idea to spend a couple of hours with a native speaker, talking about all kinds of things. Try to go beyond casual chit-chat. But your real preparation for the test will be all the exposure and study you have gone through in the past.