Isn't Officer Selection where candidate for officer training are selected, not after the full training? According to this link it looks like a ~5 day process:
I think the Army likes it because it reinforces the idea that Army officers are an elite, and it exposes more future leaders to a little bit of Army life, so they're more positive about the Army when they make decisions when they're running a business or a government department.
http://news.bbc.co.uk/1/hi/8359547.stm
[NB Not sure I approve of people using up forces resources just to pad their CVs]