Start with the head down, chin on chest. Arms are fully extended. Next, pull up bringing your head as far back as possible, touch low sternum area to the bar. Your torso should be as parallel to the floor as you can get it.
NEVER activate hip flexion during the pull up. This is the biggest mistake I see guys make. When you activate the hip flexors you end up losing the latissimus dorsi contraction. You use chest and biceps instead of the lats-- this is not a pull up. YOU HAVE TO keep your glutes contracted eliminating the hip flexors from firing. Keep your feet back at all times during the pull up. When performed correctly you should feel a stretch in the quadriceps. The stretch confirms that the lats are in control and the hip flexors are not activated.
Hold peak contraction for about 1-2 seconds. Return to the starting position and repeat.