In the Alekhine Defence, contrary to the classical methods of playing in the opening, Black does not fight for the centre with his pawns, but begins to exert immediate pressure against White's centre. Black's knight on f6 attacks the pawn on e4, and if it advances, then Black's d-pawn joins into the attack against it.
The Alekhine Defence is particularly applicable in encounters against players who are inferior in class, as well as in games with a short time-control. This opening is not used so often in practice, so your opponent might lose plenty of time to recollect the opening theory. That might prove to be a very negative factor for him in the forthcoming fight.