The R&A and USGA place competition limits on drivers in an effort to contain distance by limiting the Coefficient of Restitution (COR). If the face on your driver flexes past 0.83 then it's too hot.
But with the JPX850 Mizuno have created an iron that breaks those rules with a face that flexes past the limits thanks to the Power Frame.
The Power Frame is designed to stretch out the areas that weight can be moved to and therefore enlarge the part of the face that contributes to increasing the COR.
The structural support from the frame has allowed Mizuno's designers to put a thinner face into the JPX850 and a thinner face means more speed transferred to the ball at impact and therefore more distance.
Mizuno stress, however, that the JPX850 is not an iron created for distance at all costs. The Power Frame increases the overall spring of the face but also the spring across a large area for long and consistent distance.
Feel has not been sacrificed for distance either. Generally, if distance is improved then feel tends to go down, but the Power Frame has been designed to allow both feel and distance to live in one place, with the short top frame edge playing an important role in controlling vibration.
Playability is also enhanced by the Dual Relief Zone found on the sole where the cut away back edge makes the sole effectively play narrower and maintain a higher level of workability than is traditionally found in this style of iron.
Tetsuya Kanayama, R&D Manager, outlines this philosophy for the JPX850:
Simply increasing ball speed is not our goal - making it happen within a manageable head size is the real challenge. With the Power Frame we were able to take ball speeds to a completely different level - but keep it hand in hand with a nice feel and a level of workability"