Usually chess authors are preoccupied to explain the moves in their games, but hardly anybody explains all existing chess principles that rule the game of chess.
It took me a long time to get aware of them myself... Usually the ordinary club player has never heard of them and for that reason does usually not follow them consciously. He might make a move instinctively which obeys a chess principle but this is more or less a coincidential act and not the result of his own free will.
Black's unusual move 2...Bg4 leads to a very seldom played opening system called the Pribyl/Wade/Rat/No-name System. Get your opponent thinking right away as he cannot rely on his theoretical book knowledge that he may have, if you play a popular opening system like the Pirc (2...g6).
...the ideas above are taken from the excellent book "1...d6 move by move" from Cyrus Lakdawala which has left a deep impression on me.
The correct move here is 1...e6 and not 1...Qa5+ because 1...e6 recaptures the pawn faster as it develops the bishop at the same time. If you move the queen then you don't develop your kingside and run behind in development time.
After 1...e6! White will not be able to protect his c-pawn with 2.b4 because after 2...a5 his pawn structure will get weak and Black regains the pawn in any case.Example: 1...e6 2.b4 a5 3.c3 axb 4.cxb b6 and soon the pawn structure will collapse. See below
...to be continued...