Significant here is that the entire operation by which the friction discs and 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 {HEAD ~ the operator} having to move the crowd handles, or readjust {TAIL ~ the ultimate setting} of the piston pressure 
to readjust
Significant here is that the entire operation by which the friction discs and 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 { HEAD ~ the operator } having to move the crowd handles , or readjust { TAIL ~ the ultimate setting } of the piston pressure
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