Help with this Lamda Expression

Absolutely Not.
 
You have misunderstood the functionality.
 
From the word itself, you should understand how it works.
 
TakeWhile --> Take the words Until the condition satisfies.
 
From your example, its iterating starting from 5.
 
For 5 --> Condition is true
For 4 --> Condition is true
For 1 --> Condition Fails. So its stop printing after 4. It wont proceed further.
 
Thats why the output 5, 4
 
Hope you understood. Let me know if you still need clarification.
 
Another example for you.
 
 string[] fruits = { "apple", "banana", "mango", "orange",
                                  "passionfruit", "grape" };

            IEnumerable<string> query =
                fruits.TakeWhile(fruit => String.Compare("orange", fruit, true) != 0);

            foreach (string fruit in query)
            {
                Console.WriteLine(fruit);
            }

            /*
             This code produces the following output:

             apple
             banana
             mango
            */

Here the above program prints the fruit name until it matches the fruit name orange. Its a cool functionality and its more useful in iterations.
 
You can use it in a situation like, you can do the looping until the condition satisfies. Its just a single line of code. Thats the Functional Programming.