The fork engages the base of the bar and then proceeds to slide up the bar as {HEAD ~ the forward motion} of the bar causes {TAIL ~ the arm} to pivot 
The fork engages the base of the bar and then proceeds to slide up the bar as { HEAD ~ the forward motion } of the bar causes { TAIL ~ the arm } to pivot
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH