The function replaces the second character of the input string with a character that is one position before the original character in the alphabet. If the second character is 'a', it doesn't get replaced.

[CODE]:
```python
def approximate_function(s):
    if len(s) < 2 or s[1] == 'a':
        return s
    return s[0] + chr(ord(s[1]) - 1) + s[2:]
```

Let's test with a few more inputs to validate the description.

COMMAND: PYTHON(./temp/function.py "table" "chair" "apple" "programming")