I have been trying to remember the word, in English, used on Gower for sheep on back, too wet, fat, over-fleeced or pregnant to get up. 'Cast' rings a bell, but I'm not sure there is a simple translation into Cymraeg. In fact Ifan Jones Evans comments implies there is not!
edit: p.s. I have turned such sheep over many times, once with added difficulty when my 'Auntie' wanted to help instead of holding her dog away from the action as I had asked! But, once when I had three dogs with me, I met an unfortunate ewe tangled in wire against a wall. The dogs did not help and I couldn't work out how to free her, so I ran me and dogs back to the farm. Fortunately the farmer was in and came with me to..no sign of ewe. We finally found some blood on the wire to prove I wasn't going nuts, but he said it is amazing the messes sheep can get themselves out of!