The first flag was on us for not having enough players on the line of scrimmage. THe announcers later clarified it by saying that the receiver did not check off with the line judge, and that we were flagged because of this. THe annoucer said he did not know that the receiver had to do that, and I know I did not. Could anyone clarify this rule for me? I often see the receivers check, but I did not realize it was mandatory.