I frequently get the question what the trems "FLEX", "STIFF", etc means when referring to the stiffness of a knife. To get at least one answer, I sent the question to the Forschner distributor here in the US. The following is what I received.

Press blade side-ways on a table.

STIFF =	no bend in the blade what so ever.
Semi STIFF =	slight give or bend in the blade evenly through the length of the blade.
FLEX =	curved arc from tip to mid blade approximately 45 + (-)degrees
Semi FLEX =	curved arc from tip to mid blade approximately 20 + (-)degrees.
Super FLEX =	curved arc from tip to mid blade almost nearly 90 degrees.

Each manufacturer has its own interpretation of what the right specifications are. So, unless you kow who the manufacturer is and what their specs mean, then about the only way to test is to have the knife for your feel.