Halibut Gambit/Jaenisch Gambit

[Simon Rogers]

One of our first team players has started playing the response 1.b5 against the English Opening in league matches. I have not seen this before.
What are black's plans and ideas?
Replies for white aswell?
Your help will be much appreciated.
Many thanks.

[IM Jack Rudd]

It's probably an attempt to get something similar to the Benko Gambit after 2.cxb5 a6, but black hasn't forced white to play d4-d5 beforehand, so the usual pressure down the long diagonal may not materialize.

[Roger de Coverly]

One of our first team players has started playing the response 1.b5 against the English Opening in league matches. I have not seen this before.
What are black's plans and ideas?

As Jack suggests, the sequence 1. c4 b5 2. cxb5 a6 is the most popular. On a small sample 1. c4 b5 2. b3 scores well.

It could also be seen as an attempt to get a reversed Sicilian Wing Gambit (1. e4 c5 2. b4 )

I tried 1. b4 c5 once, but the reply 2. b5 seems useful.

[Nick Burrows]

On a small sample 1. c4 b5 2. b3 scores well.

This makes sense as one of the solid ways to decline the Sicilian Wing Gambit is 1.e4 c5 2.b4 b6, so here you get it with an extra tempo. A good line to play if you don't like playing messy/irrational positions.