Grip and motion
, who is sometimes credited with inventing the knuckleball
As used by Cicotte, the knuckleball was originally thrown by holding the ball with the knuckles, hence the name of the pitch. Ed Summers, a Pittsburgh teammate of Cicotte who adopted the pitch and helped develop it, modified this by holding the ball with his fingertips and using the thumb for balance. This grip can also include digging the fingernails into the surface of the ball. The fingertip grip is more commonly used today by knuckleball pitchers, like retired Boston Red Sox pitcher Tim Wakefield, who had a knuckleball with a lot of movement. There are other prominent knuckleball pitchers like Hall of Famer Phil Niekro, who had a very effective knuckler and knuckle curve, and Cy Young Award winning pitcher R.A. Dickey. However, young pitchers with smaller hands tend to throw the knuckleball with their knuckles. Sometimes young players will throw the knuckleball with their knuckles flat against the ball, giving it less spin but also making it difficult to throw any significant distance.
Regardless of how the ball is gripped, the purpose of the knuckleball is to have the least possible amount of rotational spin. Created by the act of throwing a ball, the ball's trajectory is significantly affected by variations in airflow caused by differences between the smooth surface of the ball and the stitching of its seams. The asymmetric drag that results tends to deflect the trajectory toward the side with the stitches.
Over the distance from the pitcher's mound to home plate, the effect of these forces is that the knuckleball can "flutter," "dance," "jiggle," or curve in two different directions during its flight. A pitch thrown completely without spin is less desirable, however, than one with only a very slight spin (so that the ball completes between one-quarter and one-half a rotation on its way from the pitcher to the batter). This will cause the position of the stitches to change as the ball travels, which changes the drag that gives the ball its motion, thus making its flight even more erratic. Even a ball thrown without rotation will "flutter", due to the "apparent wind" it feels as its trajectory changes throughout its flight path.
Hitting a knuckleball is different enough from other aspects of baseball that players specifically prepare for the pitch during batting practice before games they expect it in. According to physicist Robert Adair, due to the physiological limitation of human reaction time, a breaking knuckleball may be impossible to hit except by luck. If a knuckleball does not change direction in mid-flight, however, then it is easy to hit due to its lack of speed. (A common phrase for hitting a knuckleball is "if it's low, let it go; if it's high, let it fly"; meaning that a batter should attempt to hit a knuckleball only if it crosses the plate high in the strike zone.) Since it typically only travels 60 to 70 miles per hour (97 to 113 km/h), far slower than the average major league fastball 85 to 95 miles per hour (137 to 153 km/h), it can be hit very hard if there is no movement. One 2007 study offered evidence for this conclusion. To reduce the chances of having the knuckleball get hit for a home run, some pitchers will impart a slight topspin so that if no force causes the ball to dance, it will move downward in flight. Another drawback is that runners on base can usually advance more easily than if a conventional pitcher is on the mound. This is due to both the knuckleball's low average speed and its erratic movement, which force the catcher to keep focusing on the ball even after the runners start stealing their next bases. However, since a typical major league starting rotation exceeds the length of a series against any one opponent, one way a manager can mitigate this disadvantage is to adjust his team's pitching rotation so as to eliminate (or at least minimize) games in which a knuckleballer would pitch against teams with a preponderance of fast baserunners. Some knuckleball pitchers, such as Hoyt Wilhelm and Tim Wakefield, had catchers specifically assigned to them to catch their knuckleballs.
A paper presented at the 2012 Conference of the International Sports Engineering Association argues, based on PITCHf/x data, that knuckleballs do not make large and abrupt changes in their trajectories on the way to home plate—or at least, no more abrupt than a normal pitch. It speculates that the appearance of abrupt shifting may be due to the unpredictability of the changes in direction.
The knuckleball are also employed by the Indian cricket fast bowlers Bhuvneshwar Kumar and Zaheer Khan as their slower delivery. The physics of the operation are largely the same. However, the seam on a cricket ball is equatorial, and thus the extent of erratic movement is reduced due to the symmetry (at least in the conventional release position where the planes of the ball's trajectory and the seam are nearly co-planar). In addition, the lack of backspin does shorten the length of the delivery, and also tends to make the ball skid off the pitch—faster than it would come off a normal delivery.