Significant here is that {TAIL ~ the entire operation} by which the friction discs and {HEAD ~ reaction plates} are released, forward motion of the trencher stopped, the trencher moved backwards so as to clear the obstruction, the wheel restarted, and the trencher again placed in forward motion, are all accomplished without the operator having to move the crowd handles, or readjust the ultimate setting of the piston pressure 
Significant here is that { TAIL ~ the entire operation } by which the friction discs and { HEAD ~ reaction plates } are released , forward motion of the trencher stopped , the trencher moved backwards so as to clear the obstruction , the wheel restarted , and the trencher again placed in forward motion , are all accomplished without the operator having to move the crowd handles , or readjust the ultimate setting of the piston pressure
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL REL OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH REL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH