Splay Tree C++, Please helping me

[gongzhen]

Hi, I am learning how to build my splay tree with C++.
I found some source on line.
http://www.cplusplus.happycodings.com/Data-Structures-and-Algorithm-Analysis-in-C++/code79.html
http://www.cplusplus.happycodings.com/Data-Structures-and-Algorithm-Analysis-in-C++/code80.html

I pasted pieces of codes and then ask some questions.

/**
* Construct the tree.
*/
template <class Comparable>
SplayTree<Comparable>::SplayTree( const Comparable & notFound )
          : ITEM_NOT_FOUND( notFound )
        {
            nullNode = new BinaryNode<Comparable>;
            nullNode->left = nullNode->right = nullNode;
            nullNode->element = notFound;
            root = nullNode;
        }

Question:
why the nullNode's left child and right child are both equal nullNode. Can you explain for me why and how the codes works like that.