The function extracts digits from the input string and converts them to their respective word forms, separated by space.

[CODE]: 
```python
def approximate_function(s):
    number_words = {
        "0": "zero", "1": "one", "2": "two", "3": "three", "4": "four",
        "5": "five", "6": "six", "7": "seven", "8": "eight", "9": "nine"
    }
    return " ".join(number_words[x] for x in s if x.isdigit())
```