I don't see how I can be a complete bassist without knowing how to double thumb adequately as part of learning slap technique. And while I don't think I have issues with the motion, per se, my upstroke always seems weak. I've got little "meat" next to my thumbnail, so while doing the downstroke is ok, coming back up doesn't seem to strike the string with the same intensity. Maybe I'm approaching even the downstroke wrong too by striking too far forward on my thumb? I see a lot of guys' thumb bent way back when thumping or thumbing. It is bent back so they can strike the string closer to the knuckle? I guess the first question I'm asking is, on the downstroke, what part of the thumb is supposed to strike the string for slap? The second part of the question is, what is the best part of the thumb to catch on the way up so that the up and down strokes are more even? Thoughts?