When I see somebody like Mark Rubin slap like nobody's business on upright (and I have seen it lucky me), pulling or striking the strings hard enough to hit the fingerboard seems to be
essential. It's almost like using the string as a drumstick and having the fingerboard be the skin.
This may be required for electric bass slap technique too but that's definitely the goal for upright. Whatever it takes to get that, that's the key to getting that driving slap sound.
I'd almost equate slap bass with timpani to be honest.
