Much detailed information is only available in English:
The following photos all show the same Eumo male.
The Eumo is a recessive (autosomal) mutation. Eumos miss the barbules/hooklets on the flights, which in effect turns the feathers into down This makes the feathers useless for flying, so the birds can't. Fortunately, breeders have had some success in breeding Eumos that can fly better than the original birds. I would strongly discourage anyone who isn't an expert breeder from taking on this variety.