Since it doesn't look as if L. L. is going to jump in here, there are two areas inside the stummel : the larger-diameter boringwhich accepts the tennon of the stem, and the smaller passage from there to the interior of the bowl.
If the fit of the tennon face to the shoulder of the boring for it is flush, you don't have anything to clean there. But oftentimes it isn't, producing a gap that people call headspace. Smoke swirls around there in use, depositing moisture and tars.
The easiest way to check this issimply to look at the end of the stem tennon after smoking the pipe. If it's got tar on it, you've got a headspace issue.
If so, while the pipe is still warm and moist inside from smoking it, remove the stem. (Contrary to what you may have heard, it doesn't matter when you remove the stem -- what matters is when you put it back in again). Fold a kleenex in half (lengthwise), then that in half again, and soon until you've got a narrow, thick ribbon. Fold that back on itself once, and twist the end into a round shape you can work (by twisting it as you push) into where the stem tennon was. Keep twisting it in until it won't go any further. Remove it and repeat as necessary until no more tar comes out.
With an old pipe, you may have to repeat this after each outing, as more of the tar caked inside it softens from the heat and moistuire of smoking it. Eventually, you'll reach the point where you're only removing what's been deposited from its most recent smoking.
Going about it this way avoids having to make a special tool for the job.
The second area, from the tennon face to the bowl, will never be cleaned adequately by alcohol-dipped pipe cleaners. After smoking (to soften what's in there), get what you can with dry pipe cleaners, and twist in a drill bit (finger pressure only) smaller than the drilling's diameter. Pulling it straight out brings the tar that twisting it in's shaved away out with it in its flutes. Repeat with the next larger diameter bit until you're using the same diameter bit the pipe was drilled with. Don't horse it -- let the tool do the job. You don't want the tip of it snapped off inside, and you don't want to split the briar.
:face: