After 45 years of reffing football I think I understand the concept of the arm out signal to signify 11 on the field. This was also used as an aid to the wings so that when the WH signaled such, they only had to count the four in the backfield to know that there were 7 on the line. With recent rule changes, you would technically only need 5 on the line to be legal. So what signal are WH's giving if 10 or less are on the field?