I found this diagram in a book and I don't understand it. Black is to move R g6. Then white. Why can't white move Ka5 or Kb5 , to protect its pawn and still win ? 

Thanks in advance for your explanations.

if white moves to protect the pawn black takes the pawn with his rook and then if white takes black's rook on a6 black can capture white's rook, the white king is overloaded, it can't protect both the pawn and the rook.

• After 1...Rg6 2.Kb5 black plays 2...Rg5+ and the white king has to step back, else the white rook on b4 is no longer protected
• After 1...Rg6 2.Ka5 black answers 2...Rxa6+! and if 2.Kxa6 Kxb4 and only the kings are left on the board, so draw 

The key idea is that the white king can't leave his rook undefended.

hicetnunc, I would just take the pawn regardless of whether white plays Ka5 or Kb5, have I missed something? Agree Rg5+ also draws

Ahhhhhhh  , now I got it ! Thannnnnnnnnnnnksssssss MM78 and hicetnunc.

Hey, of course no you haven't missed anything : I hadn't read your post when I wrote mine and I noticed the Rxa6 idea only when I tried the second variation 2.Ka5