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