I like to call this tactic in the Ruy Lopez the "Bishop Lure" tactic, as it uses the bishop to tempt the enemy into overextending his queenside pawns. It's not game-inning, but it can sure help if you're looking to dismantle his pawn structure and stuff like that.

 

As you can see, White now has the a-file, which is semi open, handed to his rook "for free". Furthermore, he can also take with the rook(though I usually don't do that) and develop it at the same time(I don't do this because it's a heavy piece to develop in the late opening only).

Black's a-pawn, on the other hand, can be undermined and turned into a potential target when the light squared black bishop develops. 

Even better, Black is forced to play 6. ...bxa4, because other wise 7.axb5 would gain tempo and a pawn(the black a-pawn is pinned to the rook).

How can Black avoid this tactic? With a counter-intuitive knight move(the one shown above). I'm not sure whether it's a sort of sub-variation of the Norwegian Variation.

I hope this post has helped you play the Ruy Lopez as White better and helped you defend your position as Black better.

What about a4 b4 ? what then?

or 6....Bb7

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Then c3 to try to establish a center with d4. If he takes on c3 then Nxc3.

If Bb7 then axb5 and if black counters with axb5 then Rxa8, so axb5 wins a pawn for white.

the shortened title of this thread on the front page made it seem more exciting than it is

5...Na5? in your second line just hangs the pawn on e5. 6.Nxe5 Nxb3 7.axb3 and white is happy