ion's den
The Lion's den is a variation that follows after 4.f2-f4. White develops a strong centre by moving three pawns into the centre (d4, e4 and f4). Often your opponent thinks he is playing against a Pirc variation. Of course the first two moves are the same. It is also possible that white thinks 4.f2-f4 disproves the Lion. Even after the strong advance of the centre pawns the Lion still stands. After exchanging Queens black has a better position than white. This variation is a good illustration of the flexibility of the Lion.

 

1.  e2-e4    d7-d6  
2.  d2-d4   Ng8-f6    
3.  Nb1-c3 Nb8-d7    
4.  f2-f4     e7-e5 Black is not impressed by white's domination of the centre and plays on. 
5. d4xe5  d6xe5 If white doesn't take e5, black has to play exd4.   
6. f4xe5  Nd7xe5  The pawn is (forcedly) taken by the Knight. 

 

7. Qd1xd8+  Ke8xd8 White thinks it has an advantage because black cannot castle anymore.  
8. Bc1-f4  Bf8-d6 White prepares for castling on the Queen's side and attacks the Knight. Bd6 is better than Nfd7 because white can't play Nf3 anymore (Nxf3+ and gains a piece). If white plays 8.Nf3 you can't take the Knight because e4 needs extra guarding. 
9. 0-0-0  Kd8-e7 Black can also play Bd7 or Nd7 but castling is better because of the threat of Nd3+ and white loses the  bishops pairing. 
10. Kc1-b1  Bc8-g4    
11. Bf1-e2  Bg4xe2 Black controls square e5 and pressures white's isolated pawn at e4. This pawn keeps white busy. Also black's King has a better position for the endgame. 

 
Here follows a illustrative game from mister Leeuw himself;  Leo Jansen. 
P. van Gelderen - L. Jansen, 1979 ; 0-1