On pictures and in videos I see slappers having a "non-classical" left-hand posture; that is the left hand goes around the neck, almost like holding a baseball bat or something. The left hand thumb goes around the neck and touches the fingerboard and strings on the E-string-side. The classical bassist usually keep the left hand thumb more or less on the opposite side of the strings.

Why this difference? Does the bluegrass slappers use their left hand thumb to dampen the E and A strings (to avoid unwanted tones from ringing when the right hand palm hits the strings)?

//Andrés

That's why I hold it that way - to dampen the E when necessary.

I don't (or at least try not to) grip the neck. It tires you out and makes good intonation harder. But, sometimes you do it to move the bass around. Trying to put on a show, nobody wants to see you standing still. So, you're out next to the bass and not pulling it into you.

Seems I've seen more than just slappers doing this too though. But I've seen all kinds of techniques.

What's the difference? Squeezing the neck is harder on your hand/muscles/tendons/etc. Squeezing the neck makes clean intonation more difficult. Squeezing the neck makes shifting positions and notes more difficult.

I started out as a self-taught neck squeezer (and experienced all the problems listed above) and after several sessions with an excellent teacher adopted what you call the classical grip. From my perspective there is NO comparison.

I admit I am not a slapper and I don't play rockabilly--bluegrass is my main thing. But I think no matter what you play adopting proper technique is beneficial.

But those of you who do not wrap your left thumb around the neck/fingerboard: how do you avoid the E and A strings ringing witout intention during slapping?

BR,

Andrés

I was classically trained in high school, so I usually keep my thumb on the back of the neck. I use my left hand on top of the strings to mute the strings and keep them from ringing out while slapping between notes. Sort of how a Mandolin player gets their chop.

Jazz and orch bassist stand differently with the upright, almost that of a v shape.
Most slap bassist stand with it as more of a T formation.
Which in turn make the left hand come at the neck differently. the bass isnt leaned agaisnt the body to help it stay upright.
Also most true jazz, or orch bassist dont have to use the thumb on the back of the neck to hold a note or position, because of the way the bass is leaned against the body, and the left hand should be slightly pulling back.
When you stand with the bass in a T shape, you cant do thisso more pressure has to be applied to the back of the neck.

But those of you who do not wrap your left thumb around the neck/fingerboard: how do you avoid the E and A strings ringing witout intention during slapping?

BR,

Andrés

How is your FB shape?
Is hasnt been flatened to much has it? it still has curve?
Focus more on slapping the string then all the strings.