Form in Crunches

Proper starting form is lying face up on the floor with knees bent. The movement begins by curling the shoulders towards the pelvis, with hands placed behind or beside the neck, or crossed over the chest.

Using the hands to exert force on the neck can cause injury, so common practice is to avoid placing the hands behind the head itself. The hands can however, form a shelf to support the weight of the head, so that the neck flexor muscles can relax during the movement.

So long as the neck remains in an extended position with the neck flexors relaxed, then the hands are not exerting excessive force and it will not cause injury.
(Wikipedia)