In cute from cutlass, how is local_tile is used?

A friend told me:

auto gA = local_tile(mA, make_shape(bM, bK), make_coord(blockIdx.x, _)); 

here, we are actually extracting this:
mA[:, :, blockIdx.x, :]

Why? Is this correct? If not, what is this make_coord(blockIdx.x, _) doing? Thank you!!!