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.
Provides stability to the punch. If it isn't lined up, wrist can be broken. Years of fully extending your elbows in punches can cause body damage. The act of halting the motion increases strength. Shoulders are square, do not reach.