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