The first generation Positron used a dual cable, push-pull system because it had no return spring, so as not to overide the ball detents of the indexing. The second generation was PPS (Positron Pre-Select) and used the special, single, push-pull wire, in conjunction with the FFS (Front Freewheel System). There were still later versions called Positron II & III that were really 3rd and 4th generations. Confusing, isn't it? Most people consider Positron a disaster and short-lived but it worked well and survived into the early 1980s.
Push-pull is actually a bit of a misnomer for dual cable systems which had been used by several other manufacturers, decades prior to Positron. Each cable pulls the derailleur, but in opposite directions, so it's actually a pull-pull or double-pull system. For the system to work properly, the cables tension has to balanced, but when working properly they are very nice as there is no need for friction plates on the lever to balance a derailleur return spring. Consequently the shifts are very smooth and require little effort. In the case of Positron, these advantages are minimized due to the initial force required to overcome movement out of the index detents.