I have been puzzled on my computer's suggestion of 7...a6 ( diagram ) in the Euwe defense, occuring after 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Nxf3 e6 6.Bg5 Be7 7.Bd3
Houdini seems to like this rather ugly-looking move that neglects all development.
It goes even further than that. After 8.00, Houdini suggests at depth 20 8...Nc6 with a whopping -0.23 score ( that is : in black's favour !!! ). Hmmm.
Let's look at the standard approach of 9.Qe1 after 7...a6 8.00 Nc6. Houdini gives 9...Nxd4 and if now 10.Qh4, black prevails after both 10...c5, 10...Nc6 and 10...Nxf3+. The reason for this is simple : black has not yet castled, making the white queen on h4 look rather ridiculous.
So now I start understanding. 7...a6 is a non-committal waiting move, waiting for white to show his intentions first.
So white should refrain from playing commital moves himself, and develop quietly with moves that leave black also in the dark, eg 8.Qe2 ( diagram )
8...Nc6 is now answered by 9.000 an white has everything he hoped for. As an exmaple 9...00 10.Rhe1 ( diagram ) and white is doing fine.
So dont show your cards too quickly when the defender tries to play smart !