Few situations benefit more from a "free" out than one with runners on first and third bases. A properly executed third-to-first pickoff move can help kill an offensive rally and get a pitcher out of a jam. This move is the most advanced of the picks described thus far and requires patience and understanding on the part of both athlete and coach. In baseball circles, this move is often referred to as a 31("thirty-one") move.
from Coaching the Beginning Pitcher: Chapter 8 / Picks - By Dan Keller