To properly execute a live ball appeal, a fielder must, with a live ball, tag the runner or base in question and communicate to the umpire what the infraction was and which runner committed the infraction. Such communication may be non-verbal, implicit, or assumed--so long as the intent of the fielder is clear to the umpire. Contrary to popular belief, an appeal out is not a force out unless it is regarding a missed force base. For example, if a runner from third base tags up and scores, but a runner from second base leaves too early, failing to tag up, and then is put out on the appeal, the run counts if it was scored before the appeal by the fielders. Rare situations with a viable appeal on a runner who misses his force base require the umpire to recognize an apparent fourth out.
An appeal is legal if the fielder
Umpires will only rule on legal appeals. A potential appeal is viable if the appeal is legal and the umpire knows that the runner has indeed committed an infraction and will be called out if the appeal is executed by a fielder.
The fielders now suspect that the runner left third base too early and also missed the plate. Suppose that a fielder, with the live ball, touches third base and tells the nearest umpire, "I think he left too early." This is a proper legal appeal, and the umpire should rule with a safe signal, perhaps saying "No, he was fine." Now no legal appeal may again occur on that runner at third base. Suppose then that a fielder, with the live ball, touches home base and says to the nearest umpire, "I think he never touched home." This is a legal and viable appeal, and so the umpire should call the runner out and direct that his run shall not count.
Since the ball was live (and indeed must be for appeals to be legal), the runner from first could have attempted to advance at any time during the appeals. If the defense attempts to play on that runner, their opportunity to appeal the runner from third base is lost, and the run would count regardless of any subsequent attempt to appeal.
When a batter appears to have swung at a pitch, but the umpire-in-chief calls it a ball, a member of the defensive team (by rule the manager or catcher, though the pitcher often appeals and is usually recognized) may appeal for information from a base umpire with a better view of the pitch on whether the batter swung. The field umpire then signals whether the batter swung, and such a judgment must, by rule, prevail. The umpire asked is usually the first-base umpire (or third-base, if the batter is left-handed). The umpire-in-chief is not required to ask for assistance if he believes the request is making a farce of the game. If the bat barely left the shoulder of the batter and the catcher appeals the "no-swing" call, the umpire will probably deny the appeal. This procedure was introduced because it is commonplace for an umpire-in-chief to be unable to see some swings. A manager may ask an umpire to request assistance on other plays where another umpire had a better view, but the umpire is not required to do so. Such requests are common when a close home run or foul ball call is disputed, or when determining the accuracy of a close catch or no catch call.
An appeal may be executed if a fair ball becomes dead by leaving the playing field or becoming unplayable (home run, ground rule double, wild throw into stands/dugout, stuck in fence) if the defense believes a baserunner failed to touch a base before touching the next base to which he is entitled. For example, if the batter hits a ball which goes over the outfield fence in fair territory (whether a home run or ground rule double) but fails to touch first base before touching second base, he may not return to first base to correct his mistake once he has touched second, and he is out at first base if the defensive team appeals. In such a case, the plate umpire would put a new ball in play, and after the ball became live by rule, the defense could appeal the missed base. Again, the ball must be live.
In U.S. high school games or other games governed by NFHS rules, the defense may execute any of the live ball appeals above during a dead ball by simply communicating the infraction to the umpire, so it is never necessary to attempt a live ball appeal; it is always safer for the defense to ask for time to make the ball dead, and then make any requests to the umpire.