A swerve is related to a hose, in that the question punishes players who buzz in with knowledge of the answer. Unlike with hoses, the player is not the victim of blatantly wrong information (or information that "uniquely" identifies multiple answers); rather, the question "swerves" to a new direction by asking something tangentially related to the rest of the question.
Swerves are considered anathema to good quizbowl because they specifically inhibit players with knowledge from buzzing and/or punish players for not waiting until the end of the question.
Types of Swerves
The left turn is the classic type of swerve in which the question literally "turns" from one subject to another in the middle of the tossup. This is often caused by failure to correctly use or to misleadingly use pronouns. For example:
One man gained fame for starting the Kulturkampf as well as a war with France. (*) Otto van Bismarck lends his name to the capital of which U.S. state?
ANSWER: North Dakota
A player may buzz in at or before the (*) mark with "Bismarck", since the question seems to be talking about a man and not a state. The net effect of these kinds of left turns is to confuse and punish players without educating them or rewarding anything more than luck and mind-reading skills.
Unclear Answerline Swerve
This type of swerve makes it entirely unclear to the player as the question is being read what exactly is going to be asked for by the end of the question. Given that the player cannot see the whole question, the player may attempt to buzz in earlier. For instance:
...Tobacco is obtained from the leaves of the tobacco plant, which may be affected by a certain virus, (*) the Tobacco Mosaic Virus. From what plant is opium taken?
A player buzzing in at the asterisk might well think that the question is going to ask for Tobacco Mosaic Virus, but instead the final sentence swerves to a different question.
Irrelevant Information Swerve
This swerve type includes some kind of clever comment or irrelevant information before suddenly swerving to ask a question. For example:
...Looking both ways before you cross the street is a good idea. Looking both ways before you cross a railroad track is also a good way to exercise caution. Spell "caution."
The first few sentences are utterly irrelevant to the question that is actually going to be asked until at the last moment the question "swerves" to a spelling question (which is also usually not desired).