The GraphQL validation using directives allows developers to add validation rules directly to their Schema Definition Language (SDL) using a new module called graphql-constraint-directive. This module exposes a `@constraint` directive that can be used to decorate SDL fields with validation rules, such as formatting and length constraints. With this approach, developers can easily see their validation logic in the schema and avoid writing repetitive code in resolver functions or models. The `@constraint` directive wraps each field with its own scalar type, allowing for more flexible and user-friendly error messages. This approach also enables the creation of custom directives to validate and manipulate field values, making it a powerful tool for building robust and maintainable GraphQL APIs.