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If all the P& had was the spring, then the P7 would be "blow-back."  The round fires from a barrel fixed to the frame, the bullet flies one way, the cartridge flies the other way and pushes the slide back.  Then the spring pushes the slide back.  This works fine for small caliber or lower power pistols.  Most pistols up to and including .380 Auto are just blowback, no exceptions I can think of.

You can make a 9mm Parabellum blow-back pistol.  One option would be a big heavy spring that would be stronger than you, so you couldn't cycle the action.  The other approach is a big, heavy slide or bolt.  The 9mm Tec-9 has a 3 pound bolt, and fires blowback.  The Saturday Night Specials from HiPoint are straight blowback up to 45 ACP -- and weigh a ton, big heavy slides.  About $100 and a bargain at half the price.

So when you cross the 9mm line, you normally use a "locked breach."  This is some sort of system in which, firstly, the barrel is no longer mounted to the frame.  It jiggles around inside the slide and that makes the gun less accurate.  The barrel and slide are locked together when the gun fires (locked-breach).  They slide back as a pair, locked together in recoil as the bullet flies down the barrel.  Then *after* the bullet has left the barrel and the chamber pressure is dropping away, the barrel unlocks and stops.  The slide continues all the way back from inertia, then the spring brings it forward, it picks up a round, then locks with the barrel, and the pair come back full forward.

Keeping the action locked until the chamber pressure has dropped greatly reduces the amount of recoil, and it puts that energy into the bullet.  All you have to do is figure out how to keep the chamber locked for a bit, till the bullet leaves the gun.  Beretta 92's, old Colt 45's, Glocks, and all copies have the barrel locked into the slide with ridges or a block or whatever.  They recoil as a pair for about 3'4" of an inch.  Then some link or grooves or ramp yanks down on the breech end of the barrel, it disengages from the slide, and stops.  Just hand cycle one and watch the barrel move. 

There are many other ways to lock the breech.  Beretta Cougar's use a rotating barrel and a cam.  Calico 9mm pistols and carbines, and the H&K P9 pistol use locking rollers.  An M1 Carbine uses a rotary bolt.  And the P7M8 uses a gas piston.  As the chamber pressure builds up it leaks through a small hole into the piston cylinder. This pressure on the piston retards the motion of the slide backwards.  Since the piston is smaller than 9mm, the force from the cartridge will always be higher.  In the real world of mechanisms working that fast, it basically keeps the slide still till the round leaves the barrel.  Then the chamber pressure does win out, and the spent brass starts moving the slide backwards.  The piston does continue to cushion the whole stroke.

So, why this way?  First, the barrel is fixed to the frame.  You only have to worry about the slide to frame wobble, which makes the gun more accurate.

Next, you can use a smaller spring, which makes the pistol easier to use, plus thinner and lighter.

Next, it automatically adjusts to different power levels -- low power rounds won't fail to cycle -- +P+ rounds won't slam.  You have to change springs on a Beretta 92 to do this.

And then the most important reason -- you can't "limp-wrist" the P7.  You ever watch cop videos where the officer has to shoot?  About half of them have their gun jam after the first shot.  If you look, you will see in their panic at being run down by a guy in a car, etc., they do not hold the gun firmly at full arms length with their wrist muscles locked tight.  They just pull the trigger out of panic and the gun jams.  Locked breach guns like the 92 and the Glocks must be held firmly so that when the bullet goes one way and recoil shoves back on the locked barrel+slide, you hang onto the frame and *only* the slide+barrel move back.  If you don't hang on the whole gun will recoil backwards and the action might not even open.  That is "limp-wristing", and it will jam every gun.

But it won't jam any blow-back gun.  They run mainly on gas pressure, that brass wanting to come out of the chamber, shoving at the slide and pushing it back.  In a locked breech gun that force just pushes against a locked surface and does nothing.  Take note, the TEC-9 is always called a "junk" gun by those who say such things.  But as a blowback gun, it *never* jams.  I've never seen a single jam in mine or anyone's.  It may not be target-practice accurate -- but it don't be jammin.

And you cannot jam a P7 by limp-wristing it.  It doesn't count on recoil operating the slide -- once the piston pressure starts to drop away it is just another blow-back gun.  Check out this story about the NJ State Police.  They gave them new S&W pistols to replace their P7's.  These guys had never had to hold a gun as tight and firm as all the other 9mm pistols, they had trained with their P7's.  The malfunctions!  Read the story.