That system hasn't cracked SHA1, it seems to be a comparison system...

If you go here and create an alphanumeric string with lower and upper case characters say about 6 characters long and make an SHA1 hash of it then try to decrypt it with the one on undoMD5, it will more than likely not be able to to it. But if you first enter those same characters into undoMD5; create a hash of it and then try to decrypt it, it will find the characters.

Thats all it does is search for the entered characters and bring back the hash from another field. The hash was created and stored in the database along with the string you wanted to encrypt when you first created a hash, so when you go to decrypt the hash thats all it does is search for the hash and bring back the plain text characters for that hash. It's hardly "decrypting" anything, it's kind of false advertising;). The more plain text characters entered on that site the better it becomes and finding the hash.