How does [branch] get executed in HLSL?

The [branch] attribute can mark an if statement in HLSL to make it execute only one branch instead of all branches and discarding the results like when using [flatten].

My question is how can this actually work, when a branch diverges withing a warp/wavefront? As far as I know, in this case all threads must execute all branches taken by any of the threads in the warp (like when using [flatten]) which is consequence of the fact, that they are all within the same SIMD block and must execute the same instruction.