Help with Chuck Rainey style slap + up and down finger

[Waabs]

Sorry for the long muddled thread title. I saw Chuck Rainey doing this on a video where he was slapping and in between slaps using his index finger almost like a pick. I just can't seem to figure out exactly what he was doing and how he was doing it. Whenever I try to do anything like that it doesn't seem anything near right!

Can anyone give a simple detailed explanation of the use this technique combined with slap?

Thanks for any help.