what are flexing wings?
If you are referring to wing flex that is common on about any large aircraft. I don't know if you've ever seen a B-52 sitting on the tarmac and then in the air but they are a great example of wing flex.
If you've ever been up in a commercial airliner in rough air you will see wing flex. You can look out the window and see the wing tips flex up and down.
On the C-17, you know how the wings are drastically bent down, is this wing flex or is it built this way?
I think a *New great example of wing flex would be the Boeing 787. The wings are due to flex as much as 30%
To try to answer the original question, wing flex is an engineering characteristic of the wing that allows the wing to droop, sometimes noticeably as with the B-052, when the wing is under no or low lift conditions, to rise to or above horizontal when the wing is in greater lift conditions, and to move up and down within allowable ranges when the wing is being buffeted.
I'm not an engineer but I think I can give a simple explanation for the reason. Rigid structures have to be built with considerably more strength, and therefore weight and cost, to be able to withstand heavy stresses. Skyscrapers are built to sway to a certain degree in either high wind or earthquake conditions. If they were rigid, and built with the same materials and strength, they would topple in heavy stress. Lifting bodies, including wings, are the same. They need that flex to withstand the rigors of constantly changing lift and turbulence induced buffeting. Otherwise, they would have to be costlier and heavier to survive, robbing the aircraft of additional payload capability.
There's no material known which can accomplish totally rigid wings on for example a B52. One can come close, of course, by making the structure VERY thick, stiff and heavy, to such an extend that to the unaided human eye they don't seem to flex. However, a B52 build like that would not only cease to look like a B52, but it also wouldn't manage to get off the ground. It might even collapse under its own weight, just sitting on the tarmac.