And {HEAD ~ a rotary drive torque} of a level less than said maximum engagement torque but greater than {TAIL ~ the reduced engagement torque} is operative to impart an incremental thread advancing rotary drive, during such intervals as the engagement torque is thus reduced 
And { HEAD ~ a rotary drive torque } of a level less than said maximum engagement torque but greater than { TAIL ~ the reduced engagement torque } is operative to impart an incremental thread advancing rotary drive , during such intervals as the engagement torque is thus reduced
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH