Ef mount version 3 combines the best of two worlds. The locking mechanism of an EF mount and the sturdyness of a PL mount.
Version 2 had a postive lock which gave increased stability and no lens wiggle due to the wiggly nature of the EF mount. Only issue with version 2 is that you have to use two hands to attach the lens. Version 3 combines a standard EF rotating lock, where you rotate the lens inside the lens barrel with the extra add on of a tighting ring. So you can attach the lens with one hand in the same manner as you do on a standard canon EF mount, but by tightening the ring, you get the about the same sturdyness as a PL mount, so you dont see any lens wiggles and can use matteboxes and follow focus motors without any displacement of your lens!