If the switch fails open, {HEAD ~ the potential} at the load side of the switch will drop, and the second diode will allow {TAIL ~ current} to flow to the load through the circuit 
If the switch fails open , { HEAD ~ the potential } at the load side of the switch will drop , and the second diode will allow { TAIL ~ current } to flow to the load through the circuit
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 OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH