That "slap" on the downbeat is from the string snapping against the FB - not slapping with your hand. The string is plucked so it snaps back against the FB on the down beat, creating the initial "click" and note, then followed with two hand slaps, creating the two additional clicks... Sounds like "Doon-ticka, doon-ticka, doon-ticka..."
Be sure to check out
www.rockabillybass.com as well... Tons of info geared primarily to the slap bassist.