Forarm rubs against your side as your fist moves forward. Prevents winging of the elbow. Provides extra power when friction ends. Fist is palm up until last possible moment, then twists to final position. Twist provides extra power.
Fist is loose until last possible moment, squeeze on impact.
Relaxed muscles move faster, but actual impact should be done with solid fist. Teaches you to turn your fist over. See advanced punch technique below. Wrist is straight. First knuckle is lined up with forearm.