I watched a video on YouTube where a guy (Richard Haw, a member here) disassembled one of these lenses. It is clear now that the feature in question is not a button at all but a spring housing. There is a spring and small steel ball under it that, when the lens hood is extended, kinda softly locks the lens hood in place. Its not really a lock at all because, naturally when the lens hood is pushed back, the spring allows the little steel ball to pop up out of the channel around the rim of the lens.
mmmm...thar be a lotta words for a simple idea. Hope that makes sense.