It means when you press x (I think that's the default) you switch it from a 1her into a 2her.
This is correct, however, unlike other weapons capable of this the Langes Messer is first and foremost a 1hander, so you do not suffer penalties when using it with a shield. This is not the case with other swords like the longsword, bastard sword, or weapons like the morning star etc... Overall, the 2h mode is pretty gimmicky, and with its short length it's not useful. If you own the sword, you probably are 1h, and are better off keeping it in that mode. I'd gladly trade the secondary mode for more damage, length or speed.
Poll