Here is how a UGF works normally. There is a raised try on the bottom of the tank. Then gravel over top of it and uptake pipes to which a pump is attached.
In a normal UGF the pump pulls water through the gravel, into the tray then up the pipe and back into the tank. The gravel is the filter medium with these filters. The issue with running a UGF the normal way is that it pulls waste under the tray where it is hard to remove.
With a reverse UGF you just flip the pump around so water goes down the pipe, into the tray, and is then forced up through the gravel and back into the tank. IMO it seems much easier than running a ton of pipes under the gravel and should actually provide a better coverage to remove all dead spots.
Sand may work with the pipe method, with a reverse UGF it won't as the sand will go through the bottom tray. You could try laying some fine mesh on top of the tray to try to keep the sand out of it...