The function reverses the input string and then increments the ASCII value of each character by its position in the reversed string, ignoring non-alphabetic characters.

[CODE]:
```python
def custom_function(s):
    rev_s = s[::-1]
    new_s = ""
    for i, c in enumerate(rev_s):
        if c.isalpha():
            new_c = chr( (ord(c) - 65 + i) % 26 + 65) if c.isupper() else chr( (ord(c) - 97 + i) % 26 + 97)
        else:
            new_c = c
        new_s += new_c
    return new_s
```

You can try this code for different input strings and compare the results with function.py. If needed, we can further test the original function with more input strings to validate our understanding of its behavior.