411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...c6/2. d4/2...d5/3. Nc3/3...dxe4/4. Nxe4/4...Nf6/5. Ng5
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
Caro-Kann Defense, Classical Variation: 5. Ng5?
This move is not good for White; moving an already-developed piece, the queen knight, for a third time. Black already has an advantage.
However, many players with White, especially followers of Witty Alien or even Eric Rosen, usually have a sneaky idea with this move; after the natural 5...h6 by Black, White intends to sacrifice the knight for the f7 pawn with 6. Nxf7??, entering the dubious but incredibly scary Alien Gambit.