Tension takes speed and quickness
right out of the process.
This time completely relax your arm and rapidly tap your fingers on the table. Much quicker this time? That's because there was no tension. Your muscles used to tap were calm and able to perform at their fastest speed. Incidentally, those same muscles are important in the process of throwing and hitting.
When the mental side of a player feels fear, anxiety, stress, and/or worry, the physical side often reacts by tensing up parts of the body. Many times the player is unaware that this is even occurring. One major goal of learning and training the mental side of the game is for players to better recognize when this is happening, what parts of the body are being effected, and most importantly, what to do about it.