I don't believe you'll find any "lockup" function as part of the BW design. Here's my recollection of push-start capable automatic transmissions...

Nearly all early (50's and 60's technology) automatic transmissions had a "rear" hydraulic pump, that AFAIK was only there for one reason... to enable push-starting the car. During a push start, the rear pump was driven by the driveshaft, pressurizing the tranny hydraulic system, enabling the clutches/bands to engage, etc. Of course, this typically required 25-45 mph push speeds, depending on the transmission involved. This was probably not due to the limitations of the aforementioned rear pump, however. I would suspect the limiting factor would be torque converter efficency, or lack thereof, while running "in reverse", during a push-start attempt.

At some point, I assume at least partially because real bumpers (fenders) disappeared, the rear pump was eliminated from virtually all automatic transmission designs intended for passenger car use. But the Volvo BW was an older design that happened to include a rear hydraulic pump, ergo the ability to push-start the car.
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Gary Learned - '71 142E ITB / '73 1800ES / '02 S60 T5