Poisoned Pawn Variation

London System

1.d4 d5 2.Nf3 Nf6 3.Bf4 c5 4.e3 Qb6 5.Nc3

Black plays ...Qb6 at once and White leaves b2 hanging with Nc3.

The idea

After 4...Qb6 5.Nc3, White develops instead of defending b2. If Black takes with 5...Qxb2, White plays 6.Nb5, threatening Nc7+ and getting a strong initiative for the pawn.

Sources describe the queen grab as playable but murky; Black often prefers to keep developing.

Plans for White

If Black takes on b2, attack: Nb5 with the threat Nc7+, Rb1 to chase the queen, and quick development. If Black doesn't take, White has developed a piece and can continue normally.

Plans for Black

Either take on b2 and defend precisely against Nb5, or decline with a developing move and keep the pressure on d4. Grabbing without care can leave the queen trapped or the king exposed.

Watch out for

For Black, c7. After 5...Qxb2 6.Nb5, Nc7+ forks king and rook, so Black must deal with it at once.

For White, giving the pawn without an attack. If Black's queen escapes and Black develops safely, White is just a pawn down.