Watch the Double Thumbing online guitar lesson by Matthieu Brandt from Modern Mountain Banjo: Clawhammer & Up-Picking
In a regular "bum ditty" groove, you play the pedal string with your thumb and it sits on the off beat of 2 and 4. Double thumbing is the technique where you play that pedal string on every off beat. So also on the 1-and and the 3-and. This will give you more options in your grooves and works especially well in higher tempos. Mixing this up with drop thumbing and leaving out the expected pedal notes will give you a lot of new choices for your melodies too. We'll play these exercises in G sawmill tuning, meaning DGCD with a pedal G.