The function reverses the input string, but maintains the first and last character at their original positions.

[CODE]: 
```python
def fx(input_string):
    if len(input_string) > 2:
        return input_string[0] + input_string[-2:0:-1] + input_string[-1]
    else:
        return input_string
```

Please let me know if you need further clarification or more test cases.