From NCAA rules:
Targeting and Making Forcible Contact
With the Crown of the Helmet
ARTICLE 3. No player shall target and make forcible contact against an
opponent with the crown of their helmet. The crown of the helmet is the top
segment of the helmet; namely, the circular area defined by a 6-inch radius
from the apex (top) of the helmet. This foul requires that there be at least one
indicator of targeting (See Note 1 below). When in question, it is a foul. (Rule
9-6) (A.R. 9-1-3-I)
Note 1: “Targeting” means that a player takes aim at an opponent for purposes
of attacking with forcible contact that goes beyond making a legal tackle or a
legal block or playing the ball. Some indicators of targeting include but are not
limited to:
• Launch. A player leaving their feet to attack an opponent by an upward
and forward thrust of the body to make forcible contact in the head or
neck area.
• A crouch followed by an upward and forward thrust to attack with
forcible contact at the head or neck area, even though one or both feet
are still on the ground.
• Leading with helmet, shoulder, forearm, fist, hand or elbow to attack
with forcible contact at the head or neck area.
• Lowering the head before attacking by initiating forcible contact with
the crown of the helmet.