I know how the process works because of my experience and, more importantly, the experiences of many other people on forums like this one. Keep in mind that enough people (myself included) have ordered their CAIPS files that we aren't just seeing the process from the outside. We know when our file was assessed (not just when we received IA or PPR), we know when Ottawa processed our medicals, we know when the VO approved our medicals, and so on.
If you meant the reason for the new process, CIC's own letters say they are sending medical requests earlier to speed up processing. The letter I received with my (pre-IA) medical request said:
"Enclosed you will find instructions for a medical examination. These are being issued at this time in order to expedite the processing of your application. They are issued 'at own risk', in the sense that you will not be reimbursed for expenses if later your application should be withdrawn or refused."