Position on Plyos

From CrossFit.com

In the picture on the left, Tom Crubaugh is looking down at the box during his box jump launch. This causes a reflexive loss of lumbar extension that impedes sound mechanics of the jump.
In the picture on the right, Tom is looking straight ahead which allows for full lumbar extension and consequently powerful hip extension and an efficient jump.

In addition to the postural reflex error, head down postures (looking at your feet) motivate a disruption in proprioception generally and balance specifically. Most well directed gross motor movements require longer sight lines and more distant visual targets. Train yourself to look towards the horizon and trust your peripheral vision for the acquisition of important local targets.

On the subject of postural dynamics here from Tulsa World of Gymnastics is a brief article on the importance of chest position in functional movement.