The output of the NAND gate is used as {HEAD ~ a first control signal} to set {TAIL ~ the flip flop} and to cause another circuit to activate data transfer 
The output of the NAND gate is used as { HEAD ~ a first control signal } to set { TAIL ~ the flip flop } and to cause another circuit to activate data transfer
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH