"The purpose of the string bet rule is to stop a player from “calling,” receiving information from his opponent’s reaction, then adding more chips to change the wager to a raise. If a player obviously is trying to raise the pot and puts in almost enough chips to raise, this should be treated as if he has announced a raise, and he should be required to put in the small remaining amount to make the wager correct." Bob Ciaffone
A classic poker scene that you see repeated over and over again in the movies ... a grizzled, cigar-smoking poker player of about 60 years old is sitting at a poker table. The action is on him. Another player has made a bet of $100. The grizzled old poker vet looks at his hole cards, he's usually leaning back in his chair in a cloud of smoke, and states ...
"I'll call your $100 and raise you another $500"
That, my friends, is a string bet and is illegal. You might see it happen in movies, but you should never see it allowed in a casino. You can't call and raise at the same time. You can call ... or you can raise - but you can't do both. As soon as the grizzled old player said "I'll call your $100" - he called and can no longer raise. The fact that he said "raise" just after he said "call" is meaningless - he verbally stated call before he verbally stated raise.
Robert's Rules - Section 3 -
Betting & Raising
8. A verbal statement denotes your action and is binding. If in turn you verbally declare a fold, check, bet, call, or raise, you are forced to take that action.
A string bet is a bet that is not done in one motion - it is "stringed out" or "stretched out" too long. You can't call and raise at the same time just like you can't fold and raise at the same time! You are only allowed one 'action' when the action is on you.
There are also rules about how you must place your chips into the pot when you make a raise. The rules differ, depending on whether you are playing in a cash game or a tournament.
Robert's Rules - Section 3 - Betting & Raising
6. At non-tournament play, a player who says "raise" is allowed to continue putting chips into the pot with more than one move; the wager is assumed complete when the player’s hands come to rest outside the pot area. (This rule is used because no-limit play may require a large number of chips be put into the pot.) In tournament play, the TDA rules require that the player either use a verbal statement giving the amount of the raise or put the chips into the pot in a single motion, to avoid making a string-bet.
Robert's Rules - Section 15 - Tournaments
24. At pot-limit and no-limit play, the player must either use a verbal statement giving the amount of the raise or put the chips into the pot in a single motion. Otherwise, it is a string bet.
When playing tournament poker, you have two choices when raising ...
A betting line is a line that is painted on the poker table felt about 6-12 inches in front of each player. Any poker chips that a player moves past the betting line are considered to be part of a bet and are now in the pot. Most poker tables do not display the betting line and on racetrack poker tables, the betting line is sometimes considered to be where the wood meets the poker cloth.
On tables that do not show a betting line, any chips that are moved in a forward motion towards the pot are considered to be part of a bet and are now in the pot. Every player has personal space on the table directly in front of his seat. You may place chips from your main stack into your personal space and then stack and move chips back and forth between your main stack and the chips in your personal space - but as soon as you make a forward motion from your personal space, your chips are considered to be in the pot.
Exception: A player will sometimes announce raise and then move forward only enough chips to cover the amount needed to call. The dealer will then scoop every player's call chips into the pot. The raising player must then place additional chips into the pot, equal to the amount of his raise. This 'double forward motion' makes it easier for the dealer to keep track of the chips in the pot.
Information provided by HomePokerTourney.com