Innova defines a flex shot as:
"A Flex Shot is performed by throwing an overstable disc with an anhyzer angle of release down the left side of the fairway (for RHBH thrower). The disc travels from left to right then the overstability of the disc turns the disc back to the left. It is a very common shot that pros use."
A little vague, but it doesn't seem to differentiate between a drive and an upshot. Personally, If I were throwing a drive anhyzer with an overstable disc, I wouldn't think of it as a flex shot, more just an anny. Also, I don't agree with their statement that the disc turns back to the left - it certainly can, but where this shot proves most valuable to me is when I keep it low enough, or give it enough gyroscopic stability from spin that it never comes back at all. Instead, it lands flat and skids to a stop. I guess the real question is what is the 'flex' in a flex shot. It starts out overcoming the natural overstability of the disc, then either fades back or lands flat depending on technique and intent. To me, 'flex' means basically the same as 'bend', so I think the reason it's called a flex shot is it takes a shot that would normally go straight then finish left and flexes or ends it so it behaves more like an understable disc than the stable disc it actually is. Kind of like a hyzer flip takes an understable disc and bends it into a more stable flight pattern. But that's not to say a flex shot is the reverse of hyzer flip - it's very different.