I think the idea is to swish it so that the dirt floats above the sand (which is compacted) then the gravel vac sucks it up while its floating. You dont actually get near the sand with the vac. About an inch above is ok
I personally just use the gravel vac for gravel and just use the hose for the sand,i found the dirt to heavy for the vac to suck up being 2 inches above the sand but just using the hose sucks anything up,hardly any sand and doesnt waste as much water