If, for example, {HEAD ~ the external environment} is not driving {TAIL ~ the pin}, and the device is not driving the pin, the pin test buffer will be modeled as, effectively, a large, non leaky capacitance 
is not driving
If , for example , { HEAD ~ the external environment } is not driving { TAIL ~ the pin } , and the device is not driving the pin , the pin test buffer will be modeled as , effectively , a large , non leaky capacitance
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL 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