The switch opens a solenoid valve which vents {HEAD ~ the hydraulic pumps} which move {TAIL ~ the tracks} of the trencher in the forward direction 
move
The switch opens a solenoid valve which vents { HEAD ~ the hydraulic pumps } which move { TAIL ~ the tracks } of the trencher in the forward direction
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH