2...Ne4 starts the Main Line Trompowsky Atack. Black attacks the bishop, thus forcing White to move it; the bishop usually moves to f4. Now both sides have moved a minor piece twice in two consecutive moves. Theory stretches out at this point, thus giving White and Black much freedom, which is why many players have committed this opening to memory.

Usually Black follows up 3...c5 attacking the pawn, and the Black knight is often displaced from its post on e4 with the f-pawn.

Wikipedia has related information at Trompowsky Attack.

.

1.d4 Nf6 2.Bg5 Ne4

• Nunn's Chess Openings. 1999. John Nunn (Editor), Graham Burgess, John Emms, Joe Gallagher. ISBN 1-8574-4221-0.
• Modern Chess Openings: MCO-14. 1999. Nick de Firmian, Walter Korn. ISBN 0-8129-3084-3.
• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.