When training neck flexion and extension it is most advantageous to start from a neutral position with good posture. In a neutral position the face pad is set so the athlete's head is under tension, which means the weight of the face pad is pushing against the back of the occiput before the exercise begins. 

Appropriate technique is important in maximizing results, example: in neck extension the semispinalis is most active when the exercise is started from the aforementioned position.

Neck Extension Posture

Neck Flexion is an 82-90 degree movement. The key, like extension is starting from neutral under tension and acquiring a position whereby the athlete can keep their shoulders depressed during the movement. Keeping the shoulders from elevating ensures the traps are not assisting during the movement.

Neck Flexion with Shoulders Depressed

Neck Flexion Training at West Virginia University