There are many different approaches and techniques for this one. I favor teaching it as touch dig or touch scuff (touch dig because sometimes the heel doesn’t go through to the air like in a riffle or 3-beat riff walk). Other teachers will teach it as a brush scuff or brush dig and technically I agree that’s correct, perhaps even more technically correct than touch dig or touch scuff, but I prefer to teach the first sound as a touch because almost universally students run the touch (or brush) and the dig (or scuff) sounds together. Teaching the first sound as a touch gets the students to hesitate for a split second before they make the next sound. That hesitation delivers 2 separated sounds, which is what we’re striving for.
Tip: If you’re still having trouble separating the sounds add more pressure to the touch and start it a little further back.
Riff = rf
Tap Dance Move Shown by Rod Howell