Yeah it's the swingweight mostly and the inherent power that it brings. Look at the power potential data for the Ig Pro, the 2012 PDR and the 16x19 Donnay Pro One using this tool: http://twu.tennis-warehouse.com/cgi-...mparepower.cgi
Each of these three tested frames had almost the same swingweight, so it's an apples to apples comparison. Notice that all three have pretty much the same power potential in the middle of the strings. That's a function of swingweight, which tells you how much mass is in the head of the racquet. The Pdr is stiffer, but that's an advantage only on shots hit near the tip, which is also shown in the data. Basically, the effect of stiffness on "power", meaning shot speed, is very slight. If a racquet flexed at RDC 100 it might be another story, but the difference between 62 and 72 is not so great. I have no idea why people use stiff racquets.
There might be something else going on, like local hoop stiffness - the IG Pro might have a hoop of steel, although I haven't heard anything like that - but these are the mysteries of the racquet engineers and they don't tell us about it. The fact they don't, through the marketing departments, suggests to me that any such mysteries are of little importance anyway.
Also notice that Chris at TW gave the IG Pro a pretty high power score in his review, but he gave the C10 Pro, a racquet with a "noodle" upper hoop, an even higher score. All that said, I would like to try the IG Pro; such is the allure of the mythical "powerful" frame. LOL