If you log into your ECAS and at the bottom Help section click on 'Citizenship Grant', it explains that for In Process the following is done :
Your file has been sent to the Citizenship and Immigration Canada (CIC) office closest to your home for processing.
The CIC office will complete the necessary steps for a citizenship judge to make a decision on your application. If additional information or documents are needed, the CIC office will contact you.
That leads me to believe once your file goes In process it is possibly assigned to a queue to the CIC office closest to your address waiting for a Officer at that office to open and assign the file to themselves, they then start all the checks etc.