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A helmet to helmet hit is when a player uses his helmet to strike an opposing player in the helmet. Due to the severe nature of the injuries that can occur as a result of this type of hit, helmet to helmet hits are penalties in the NFL, whether incidental or deliberate. Defensively, helmet to helmet penalties are usually flagged during the process of making a tackle. However, the penalty can also be enforced on an offensive player making a block or a defensive player making a block after a turnover.
As far as defensless reciever goes, If a defender launches himself and hits a reciever before he gets both feet on the ground and is prepared to take the hit, you will in most cases get flagged.(doesn't have to be helmet to helmet)