This principle is used to hide the internal working of an application.

The principle you're referring to is known as "Encapsulation". Encapsulation is a fundamental concept in object-oriented programming (OOP). It refers to the bundling of data, along with the methods that operate on that data, into a single unit. Many programming languages use encapsulation frequently in the form of classes.

Here are the steps of how encapsulation works:

1. Declare the class and the variables (data members) that you want to encapsulate. The variables should be set as private so they cannot be accessed directly from outside the class.

2. Provide public setter and getter methods that allow outside code to interact with the data. The setter method updates the value of the variable. The getter method reads the value of the variable.

3. The data is now hidden from direct access, and can only be accessed through the methods of the class. This is known as data hiding.

4. Now, the internal workings of the class are hidden from outside. The data can be validated before it's set, and the class can have control over what values are acceptable.

5. This way, even if the internals of the class change, the methods to interact with the data remain the same. This makes the code easier to maintain and less prone to errors.

So, encapsulation is used to hide the internal workings of an application, allowing you to change the internals without affecting the parts of the program that use the class.