The function appears to replace each English letter in the input string with the letter 13 positions later in the alphabet, wrapping around to the beginning of the alphabet if necessary, regardless of the letter case. This is also known as a ROT13 cipher. Non-letter characters remain unchanged.

[CODE]: 
```python
def rot13_cipher(text):
    result = []
    for char in text:
        if 'a' <= char <= 'z':
            result.append(chr(((ord(char) - ord('a') + 13) % 26) + ord('a')))
        elif 'A' <= char <= 'Z':
            result.append(chr(((ord(char) - ord('A') + 13) % 26) + ord('A')))
        else:
            result.append(char)
    return ''.join(result)
```