How are cuds formed on coins? [gelöst]

Hi all,
I'm really getting into coin errors like cuds & cracked dies, also rotated dies. I'd like to know please, how cuds are formed.

SORRY!! I just googled it & looked on wiki and have seen that it is usually damage to the die itself . The die has been dented or has a piece out of it .

Thanks , Merv

If you're into errors, check out https://conecaonline.org
Just in case you don't already know about it.

Thanks for the info.
I tried the web site , I was only looking for pictures but I couldn't find any. (so I gave up)