To "poll the jury" is to ask each juror to render his or her judgment verbally, usually in an open court setting. Judges may do this periodically to make certain judgments remain the same, and defendants will almost certainly ask for a jury to be polled when a verdict is not in their favor. Even though this action seldom results in anything but repetition of a verdict, a juror can change his or her mind when polled, resulting in some interesting circumstances.
One reason why a "poll the jury" request might be issued is because jury members are individuals. Each jury member is charged with the task of deciding, as an individual, what his or her verdict should be. Collective votes render a verdict, but the verdict is not supposed to be decided collectively. Instead, it should be determined by summing up individual votes, each representing a juror's best personal analysis of how he or she saw the evidence and case.
In some circumstances, jurors place pressure on one another. They might want to get home, they may feel strongly about a case, or other reasons might apply. If a judge suspects that pressure is being applied to a juror, he may poll the jury to see if, in the protection of open court, a person changes his or her mind. This could occur whether the judgment is guilty, not guilty, or for or against a defendant. Since judges are human, some may conduct a poll on the chance that judgment might go in a different direction; this could be a little pressure applied to a jury, especially if one juror is holdout.
The defendant may poll the jury too, and frequently chooses to do so if a case has been decided against him or her. This is the last ditch effort, save an appeal to be declared not guilty. Should a juror change his verdict, it allows the defendant's attorney to move for mistrial because the verdict is tainted by the juror's indecision. This doesn't mean the defendant is free, and in the rare instances where this occurs, a new trial usually follows with a new jury.
It is highly unlikely that a defendant would poll the jury if the jury vote has declared him or her innocent. A juror could still change his or her mind, which means the innocent verdict ends in a hung jury instead. Few defendants would tempt fate in this manner and risk their freedom. On the other hand, the judge can still poll the jury if he or she chooses. In exceptionally rare instances, a juror has changed his mind about an innocent verdict and declared someone guilty instead, creating the need for a second trial.