Except that this is false. The rule applies to any hits to defenseless receivers not just blows to the head. The rule, as explained by Jerry Markbreit (former NFL official) is as follows:
"All players in virtually defenseless postures are protected from unnecessary hits by the defense, which include helmet-to-helmet contact, helmet-to-body contact, and blows to the head. Intended receivers of forward passes who fail to catch the pass are considered to be in a defenseless position immediately after the pass is missed. If the pass is caught, all of these restrictions are off, unless in the opinion of the covering official, something unsportsmanlike occurs."