The two low resistance n transistors at {HEAD ~ the same time} connect the data lines to {TAIL ~ ground} 
The two low resistance n transistors at { HEAD ~ the same time } connect the data lines to { TAIL ~ ground }
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL