import sys

def most_frequent_words(filename, num_words=10):

    """Finds the most frequent words in a text file.

    Args:

        filename: The name of the file to process.

        num_words: The number of most frequent words to return.

    Returns:

        A list of tuples, where each tuple contains a word and its frequency.

    """

    word_counts = {}

    with open(filename, 'r') as file:

        for word in file.read().split():

            word_counts[word] = word_counts.get(word, 0) + 1

    sorted_word_counts = sorted(word_counts.items(), key=lambda x: x[1], reverse=True)

    return sorted_word_counts[:num_words]

if  

 __name__ == "__main__":

    if len(sys.argv) != 2:

        print("Usage: python script.py <filename>")

        sys.exit(1)

    filename = sys.argv[1]

    most_common_words = most_frequent_words(filename)

    for word, count in most_common_words:

        print(f"{word}: {count}")

```

**Explanation:**

1. **Create a dictionary:** Initialize an empty dictionary `word_counts` to store word frequencies.

2. **Iterate over words:** Read the file and iterate over each word.

3. **Update word counts:** For each word, check if it's already in the dictionary. If not, add it with a count of 1. Otherwise, increment its count.

4. **Sort word counts:** Sort the word count pairs by frequency in descending order.

5. **Return top `num_words`:** Return the top `num_words` frequent words.

**While this approach is a more manual implementation, it provides a deeper understanding of dictionary operations and sorting techniques.**

**Note:** For larger files and more complex text analysis, the `collections.Counter` class offers a more efficient and concise solution.