Weird righ hand (Finger, Slap) question...

I have a few techniques that I apply. But first I'll mention that I never rest my forearm on the bass, NEVER! The reason is, is that the muscles and nerves that control the fingers past through the forearm. When the forearm is resting on the bass we are constricting those muscle and nerves which causes the fingers to work harder. Over time this can lead to damaged muscles and nerves.

As to the thumb, I use two differnt techniques.
1) Floating Thumb Anchor
2) Stationary Thumb Anchor

Floating works this way. Playing on the G string the thumb lightly rest on the A string in alignment with the index finger. When I change strings, the position changes. Playing on D string, thumb will lightly rest on the E string once again in alignment with the index finger.
I've found this to be the most natural position of the hand.

I use the stationary thumb anchor when I'm doing a lot of string skipping. The thumb will lightly rest on one string while the fingers string skip.
I've found that I can string skip quicker with the stationary thumb anchor then with the floating. When the thumb is anchored on the e string and your playing on the G string the thumb and fingers should start to spread out. The thumb should start moving closer to the neck and out of alignment with the index finger. If the thumb is left in alignment with the index finger an unnatural hand position is achieved.

Hope this helps,

 

It is a natural position of the hand, but the stationary anchor accomodates the natural movement of a hand. Moving a finger toward a thumb is a natural movement. Resting the thumb on a string or pickup can accomodate this movement. I find that the stationary anchor works well to get a consistent sound and attack. You're less likely to unintentionally slap the string against the fretboard.

I use both. Since we all have different hands, bodies and basses, your mileage may vary with each technique.