These variations are called fills. David taught Tina how to play bass and how to play fills. David taught Tina other tricks. Like a thumb slap. These are used in funk.
Thumb slaps sound more sadistic than they really are.
Tina's thumb didn't need to do any slapping. She just had to strike the string in such a way that it slapped against the upper frets of the neck. To do this, Tina kept her thumb straight, but loose, and brought it down on the string while twisting her forearm. All these friends piling in the room. Brian Breger was from Design Research.