It seems that after 3...a6!? Black has given away his pawn for nothing, but the pawn can be regained by 4...Nb4, 5...Nf6 and then Nbxd5. Having a pawn on a6 is constructive because it denies White's bishop the ability to check on b5 which would be bad for black as white has a pawn on d5 for the time being. White's main option here is to accept black's counter gambit with:

4. exd5

.

'1. e4 Nc6 2. d4 d5 3. Nc3 a6'

Wikipedia has related information at Nimzowitsch Defence.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.