The stretch provision takes the amount owed (in this case, 3.135M) and splits it evenly over 3 years. This happens anyway in paying out the cash for 2011 CBA buyouts, but teams can choose to have the cap hit over the original contract length, or 2x the OCL plus one. So in Williams' case, the Blazers can either keep 3.135M on the cap for this year, and be done, or have to have 1.045M each year over the next 3 on their cap. So if they did that, we would have an extra 2.090M under the cap.
The 12.9M max number assumes a 58.0M cap number. Most analysts are saying it's going to be around 61-61.5M, which would make a max offer to Hibbert start at 13.6M or so.