there is nothing to do with whether the flash is powerful enough or not.
for multiply exposures, as long the background is no 0,0,0, IT WILL records some of the background when it is not mask out (block) by the moving subject during the exposures, as such, the moving subject will become translucent.
well yeah obviously because the sensor keeps recording even after the event has happened..
but you can minimise the translucency by using a nice powerful flash and keeping the shutter speed as short as possible afterwards is what i'm trying to say..