The function modifies the input string by incrementing ASCII value of each character by 2 and inserting a 'u' after 5th character.

[CODE]:
```python
def approx_function(input_string):
    output = []
    for i, char in enumerate(input_string):
        output.append(chr(ord(char) + 2))
        if i == 4:
            output.append('u')
    return ''.join(output)
```
Please note that this implementation is only an approximation of the original function and might not cover all scenarios, but it should work for most cases.