Why the following code works?

typedef char (&yes)[1];
typedef char (&no)[2];

template <typename B, typename D>
struct Host
{
  operator B*() const;
  operator D*();
};

template <typename B, typename D>
struct is_base_of
{
  template <typename T> 
  static yes check(D*, T);
  static no check(B*, int);

  static const bool value = sizeof(check(Host<B,D>(), int())) == sizeof(yes);
};

//Test sample
class B {};
class D : private B {};

//Exspression is true.
int test[is_base_of<B,D>::value && !is_base_of<D,B>::value];

Note that B is private base. Note that operator B*() is const. How does this work? Why this works? Why static yes check(D*, T); is better than static yes check(B*, int); ?