iv always wounderd how Drummers with 2 toms, get such great thick full sounding long fills with their toms. I guess it has to do with alot of bass in there as well, but its so fast it just sounds so constant and thick.
There's one way to find out: pick a specific example, and take the time to figure out what they played. Find a recording where one of these drummers does this, and then analyse it and figure out exactly what he or she did. Don't let it remain a mystery - do some homework!