illegal participation

[broncocoach]

Scouting a game yesterday I seen this and thought it was pretty cheap and illegal but i could be wrong
While on offense the coach pulled 2 players he sent 1 other player back in they hurried to the line and lined up the coach put a wide reciever on the field just before the qb went under center and there was a silent count and a quick pass to the reciever who came off the sideline of course they scored and the opposing coach went nuts the ref didn't ever see the player because he was behind the ref.
If its not illegal it cheap thats for sure