Is this similar to what Steve Baughman does on, for example, John Kelly's Reel? In that tune he does either a percussive palm slap or a middle-finger-thwack on the 2nd and 4th beat of each measure. More slaps than thwacks.
I worked on this tune and it is a good one for learning the technique I think since the melody notes played on beats 2 and 4 are all achieved via hammer-ons and pull-offs, so you don't have to worry about slapping/thwacking at the same time you are plucking a note.
Anyway, I end up using the side of my thumb for this, and usually end up hitting the three heavy strings.
It's a great technique and I think it really adds drive to a tune.
I've actually never heard anything by Don Ross - recommend some tunes/CDs of his for me to listen to please.