If you focus on using the weight of the right arm, not using the actual power of the arm. You should be able leave the right hand thumb in contact with the side of the fingerboard almost the whole time. The thumb should only be resting, not pushing against the fingerboard.
Also as already mentioned there is a turn in the wrist. . So it's this turn combined with the arm weight, not pulling your whole arm.
The weight of the arm is also used on the other strings, but you obviously don't have a string to play back onto.
Remember to keep your neck, shoulder, elbow, wrist and hand relaxed.
Hope this helps.
Geoff