The function takes a string input, calculates the ASCII value of the last character, adds the position of the last character in the string, and replaces the last character with the new character obtained from the summed ASCII value.

[CODE]: 
```python
def modified_function(input_string):
    last_char_position = len(input_string) - 1
    new_ascii_value = ord(input_string[last_char_position]) + last_char_position
    new_char = chr(new_ascii_value)
    output_string = input_string[:-1] + new_char
    return output_string
```

To further confirm this description, let's try the function on some more inputs.

COMMAND: PYTHON(./temp/function.py "example" "tricky" "verify" "solution")