This is a classic Lord of Destruction feature that is not known to all players.
During a single player game or tcp/ip or open battle.net multiplayer game, press Enter and type the following:
where X is a number from 1 to 8. Press Enter again. The game will confirm your action.
What this does is increase the difficulty of the monsters, as well as their rewards, to correspond with a multiplayer game with X players in it.
Monster life and experience is greatly boosted, monsters are more likely to drop items and their damage is slightly boosted on Terror and Destruction difficulty.
See THIS PAGE for more information about /players X