"Virtual constructor" and slicing

Can anyone explain the concept of "slicing" with respect to the "virtual
constructor" idiom as explain at parashift ?

 From parashift:

  class Shape {
  public:
    virtual ~Shape() { } // A virtual destructor
    virtual void draw() = 0; // A pure virtual function
    virtual void move() = 0;
    ...
    virtual Shape* clone() const = 0; // Uses the copy constructor
    virtual Shape* create() const = 0; // Uses the default constructor
  };

  class Circle : public Shape {
  public:
    Circle* clone() const; // Covariant Return Types; see below
    Circle* create() const; // Covariant Return Types; see below
    ...
  };

  Circle* Circle::clone() const { return new Circle(*this); }
  Circle* Circle::create() const { return new Circle(); }

In the clone() member function, the new Circle(*this) code calls
Circle's copy constructor to copy the state of this into the newly
created Circle object. (Note: unless Circle is known to be final (AKA a
leaf), you can reduce the chance of slicing by making its copy
constructor protected.) In the create() member function, the new
Circle() code calls Circle's default constructor.