The function increments the 2nd and 4th characters (if present) by two positions in the alphabet. If the character is 'y' or 'z', it wraps around back to the start of the alphabet ('a', 'b').

[CODE]:
```python
def f(x):
    output = ""
    for i, char in enumerate(x):
        if i == 1 or i == 3:
            if char in ['y', 'z']:
                output += chr(ord(char) - 24)
            else:
                output += chr(ord(char) + 2)
        else:
            output += char
    return output
```