Thunk practice is used to work on arpeggio (or scale) right hand evenness by muting the strings with the left hand. The result is that it’s really easy to hear uneven rhythms or accidental rushing/dragging. It’s a great, powerful tool.
But there’s a downside: thunk practice hides all other right hand noise. An imprecise finger placement that produces a bit of nail noise will get covered up by thunk practice.
Need to work on evening out arpeggios or scale-like alternation? Use some thunk practice (combined with changing up the rhythm), but be sure to alternate thunk practice with normal playing to keep sloppy right hand technique from sneaking in.