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