This video clip is taken from the 2015 American League Championship Series. Pitcher Marco Estrada is facing batter Eric Hosmer. Estrada starts in the windup position then transitions to the set position before delivering the pitch. This seems to be against the rules and is an illegal pitch. Since he assumes the windup position, then shifts into the set position he is clearly interrupting his commitment to pitch the ball.
The rule, 5.07.8.01[a] and [b] seems quite clear to me:
The Windup Position
The pitcher shall stand facing the batter, his pivot foot in con-
tact with 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.
The Set Position
Set Position shall be indicated by the pitcher when he stands
facing the batter with his pivot foot in contact with, and his
other foot in front of, the pitcher’s plate, holding the ball in
both hands in front of his body and coming to a complete stop.
Preparatory to coming to a set position, the pitcher shall have
one hand on his side; from this position he shall go to his set position.
The rule may be found here: MLB Rules: Legal Pitches
Obviously, if there were men on base this would be an attempt to deceive the runners and would surely be a balk. However, since no one is on base I think it should be an illegal pitch and the batter awarded a ball by the umpire.
So, what do you say? Is this an illegal pitch? Or is it legal? Give reasons for your answer.