Now you'll see an orange 'bag' floating around with a flag on it. Red flag = flax is not rotten White flag = flax is rotten This rotting might take a while, so its you have something else to do or let it rot before you offline for example.
Rotten flax can be dried to obtian dried flax. Or rotten flax can be used in a flax comb or hackling rake (10 rotten flax max) The comb or rake gives straw in the first phase, tow in the second phase, and lint in the third.