I just played it because I have watch some games with 5. b6 and I try it but I don't know what is the name of this variation.

I've seen it variously called the "Pawn Return Variation" or the "Benko Gambit Half-Accepted." (I prefer the latter, as if anyone cares about my lowly opinion.) It's more than just a play to avoid mainlines; in recent master-level play it occures at least as often as 5.bxa6, and there is a growing body of theory behind it. I would not be surprised if in the next 10 years, this becomes the most common response to the Benko.

I used to play 5.b6 when I was afraid of the Benko. Now I accept it and play the manual castle variation.

benko gambit declined. its a move suggested in a few repertiore books by GMs to avoid blacks normal massive queenside counterplay its a legit opening but white didnt play it how I have seen it suggested to play or neither did black :)

I play 5. f3

My comments aren't working well, it keeps cutting off what I write.

ok thanks for answering it's an insteresting move I think

It's a positional move.  White acknowledges Black gets a lot of play for the pawn in the Accepted (whether or not it is enough is not relevant to the thinking here), so he gives the pawn back while avoiding Black's open files on the Queenside and Black's Pa6 takes his Bc8's preferred square, and instead of having to deal with an eventual ...c5-c4, White can use the c4 square himself.

White also has the built-in idea of a2-a4-a5, so Black must plan immediately how he will recapture - he probably doesn't want to just play ...Qxb6 when a White Knight will soon appear on c4, for example.  It's not a line that leads to much of a chance for advantage for White, but it is often the type of position Black really doesn't want to play if he was hoping for a normal Benko. 

This variation is very complicated but for

Black it may be best just to immediately take the Pawn with the

Queen as the a4  a5 variations can be countered.

but I will admit not to really fully understand how to play the

Benko

Gambit even though I have played it.