I imagine it's probably this one?http://www.pipsproject.org/RenderPage.asp?LinkID=31710000
There's a short demo here (link halfway down):http://www.cemcentre.org/intu/entrance-assessment
Our son did it a couple of years ago but I got no useful feedback from him. At that time it was adaptive (i.e. the more questions the child got right, the harder the questions got and vice versa - in effect, every child got a unique test) but from the description I'm not sure if it still is. I looked for information at the time but found practically nothing and tbh I doubt you'll find very much now (unless there's some sort of underground clique of indie parents who know how to go about it...
Preparation would presumably be similar to that for a CEM written test - assuming you're doing this for an 11-yr old - so have a look under the B'ham and Warwickshire sections and in particular look for fm's posts. If you'd said he was weak on the English side I would have said the vocab CD from this site would be an absolute must, but I haven't tried the Maths resources so I can't comment.
I would say that the general sense among those who've seen the B'ham test in operation over a number of years is that it's a fairly reliable way of identifying the genuinely clever, as opposed to the methodical plodder or the heavily coached (leaving aside any discussion of the environment required to achieve the prerequisite literacy standards...).