Theoretically, that was done so hydraulic pressure to apply the clutches and bands could be developed by pushing the vehicle above 30 MPH- - - - -in case of a dead battery. As a lowly E3 autopilot tech/newlywed living in Cordell Oklahoma in early 1968 I had a '59 Chevy 283/Powerglide that didn't want to turn over fast enough to start when the temperature got down to 10 degrees or so. My "fix" for the problem was to park the car in the driveway nose out (down hill) the night before. Turn on the key, put it in low, let it roll out of the driveway and then turn right continuing downhill for a short distance. The rear pump wouldn't quite get the job done in that short distance so I rigged a bypass switch for the neutral safety switch so I could engage the starter while I reached "terminal velocity" of about 20 MPH. Started every time with the bump from the rear pump.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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