The flex is the up and down movement of the hulls in relation to each other when going to windward. Down wind with the spin the bridle lines keep the hulls pretty well in alignment.
I did find that the bolts were a little loose in spots- was able to get another 1/2 or so turn on some of them. Didn't want to over torque them though. We will see if it makes a diff. I think it will... just that level of flex didn't seem right. Hard to say but they were moving at least an inch+ or so up and down.