Final answer:
The umpire signal of crossing and recrossing the wrists below the waist in baseball indicates a balk, which is an illegal pitcher's motion.
Step-by-step explanation:
The umpire signal of crossing and recrossing the wrists below the waist is used in baseball to indicate a balk. A balk occurs when a pitcher makes an illegal motion or movement before delivering the pitch to the batter.
By crossing and recrossing their wrists below the waist, the umpire is signaling to the pitcher that they have committed a balk, and any runners on base are awarded an extra base. This signal is an important part of the umpire's role in enforcing the rules of the game and maintaining fairness.