Rubbing out a finish to achieve a perfectly flat surface on porous woods such as mahogany and walnut is a lot of work, so it’s usually reserved for tabletops. Using a film-building finish (not oil), the first step is to sand the finish back, or level it, to leave the pores filled in. You usually have to do this even if you have used a pore filler because there’s always some shrinkage. Then, when the surface is perfectly flat, use finer and finer grit abrasives to create the sheen you want.
The problem in leveling is that you can’t see how much you have progressed because the sanding sludge you’re creating prevents this. But you need to see the progress so you don’t sand through to the wood.
You can, of course, wash off the sludge, but then you have to let everything dry before you can see the shine in the pores, and this slows you down.
A much quicker way to check your progress, as long as the finish you have applied has a gloss sheen, is to remove the sludge from the area you’re checking with a plastic spreader. The glossy finish in the pores where you haven’t sanded enough shows up well.
I call this “the plastic-spreader trick.”