You may have seen it, even if you were casually watching a baseball game: A pitcher makes a small move on the mound, and then the umpire stops play and signals for the runners to move forward one base -- all with seemingly no action taking place. The television broadcasters spend a few seconds discussing a funky word called a “balk,” and then play resumes.

But what exactly is a balk? It’s one of baseball’s more intricate rules, with a long list of permutations, sometimes even leading to confusion between those on the field. But the balk essentially keeps the pitcher in check by limiting his ability to fake a pitch or a pick-off throw to one of the bases. If an umpire deems the pitcher pretended to do either, without clear intention of following through, the balk is called and each of the runners move up one base.

As complex as the balk appears, its rules come down to a list of dos and don’ts for pitchers. First and foremost for all pitchers is this rule: After they agree on a pitch with their catcher, pitchers must get to the set position -- or a complete stop with the body still and the hands together -- before starting their motion toward home plate. If there’s any fluidity between receiving the sign and throwing home that skips the set -- or if a pitcher flinches during the set -- the umpire can call a balk. A pitcher also cannot switch from the windup back to the set without fully stepping off the pitching rubber. And if their hands come apart from the set without either immediately delivering a pitch or a pickoff throw, that’s a balk too.

Now, let’s move to left-handed pitchers, those crafty southpaws who have a straight-on view of the runner at first base. Once a lefty raises the right foot, he or she must land it toward the direction in which they plan to throw. Picture a line on a 45-degree angle shooting out from the pitcher’s grounded left foot; if he or she is throwing to first, their right foot must land on the left side of that line. If they’re throwing home, that foot must land on the right side of the line.

Righties, who face the third-base side, must complete their motion once it begins -- and it must be in a clear direction toward either a) home plate or b) the base to which they are throwing. When turning to make a pickoff throw to first, righties can’t stop mid-motion unless they step off the pitching rubber. If they do, a balk will almost always follow.

Those are far from the only ways to draw a balk from an umpire. If a pitcher makes any sort of motion that mimics his or her normal delivery, and then stops before firing the pitch, that’s a balk. A pitcher also can’t throw a pitch right after getting the ball back from the catcher, nor can he or she throw one while at least one of the catcher’s feet is out of the box. They must be facing the hitter when they release the ball, meaning even Luis Tiant had to eventually turn his tornado windup back toward home plate. And the pitcher can’t reset the pivot foot or take an extra step toward home without expecting to hear a balk called soon after.

The balk made its way into the rule book in 1898, and for more than 100 years, a pitcher could fake a pickoff throw to one base before firing to another (e.g., fake to third, and throw to first) -- but that, too, was moved under the balk umbrella during the 2013 season. Perhaps the most embarrassing form of the balk is when the pitcher drops the ball -- whether it be intentionally or unintentionally -- before delivering a pitch. Yes, that, too, is a balk, and the runners advance. Opposing teams have won ballgames thanks to a game-ending balk, and it even popped up in the 1961 MLB All-Star Game, when fierce winds at San Francisco’s Candlestick Park tipped pitcher Stu Miller off the mound.

Balks can happen to even the best pitchers; Hall of Fame pitcher Steve Carlton owns MLB’s all-time record with 90 balks to his name. But it’s certainly something to avoid if you’re on the mound, and it’s a rule all pitchers should know inside and out as they rise up the ranks.