nitro rig driver:
"If reversers are only effective at high speeds then how come some use them for powerback from stands....believe DC9/MD80 type".
The answer to that is incredibly simple and I am surprised you don't know the answer already.
Most stands are constructed on an upslope of 2° or thereabouts. That is why you often have to apply power to get on stand. The corollary of course is that it makes it much easier for tugs to push back heavy aircraft since they are pushing downhill.
It therefore follows that relatively light aircraft need only take the park brake off and apply idle reverse to get off stand backwards without needing a tug.
I remember sitting in my DC-10 at Raleigh/Durham (or was it Charlotte?) watching the DC-9s backing off stand without the use of a tug. It's easy really.