The blades have a retracted position and {TAIL ~ an extended position} moving across {HEAD ~ the tunnel} 
The blades have a retracted position and { TAIL ~ an extended position } moving across { HEAD ~ the tunnel }
OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD