The test is done on computer and the results are available to school the same day.
The students get a range of questions on verbal, non-verbal reasoning, maths and processing skills. They answer the first few questions, and then the computer program decides which questions they get for the rest of the test, hard or easy as appropriate.
The results of the individual pupils are compared to a national standard for their age group, and a MidYIS score is given for each category plus an overall score.
The school should be using the MidYIS results in setting, and should track their performance in tests and exams and match them to MidYIS to see if they are doing well, as expected, or need a kick up the you-know-where.
They do offer broad GCSE predictions, eg liklihood of 5 A-C passes, ditto with English and Maths, ditto with at least two Sciences.
The tests produce masses of data that the school should be using to track performance to potential, and, less importantly, to calculate add-value scores after the GCSEs.
More info here
If you need more info, let me know. I have been doing lots of MidYIS analysis in the last couple of weeks.
At our school, we don't generally share MidYIS info with parents unless their little darling is seriously underperforming in KS4.