The windup position is described by the Major League Baseball rules as follows:
windup position
The wind-up position is one of the two legal pitching positions.
The other is the set position.
"The pitcher shall stand facing the batter,
his entire pivot foot on, or in front of and touching and
not off the end of the pitcher's plate, and the other foot free.
From this position any natural movement associated with his delivery
of the ball to the batter commits him to the pitch without interruption
or alteration. He shall not raise either foot from the ground,
except that in his actual delivery of the ball to the batter,
he may take one step backward, and one step forward with
his free foot."
