When you do the ground pound after flying and see the shock wave clear out the tentacles to the sides of the track, you'll also hear a high-pitch scream of sorts. As you do more shock waves in a row, these will rise in pitch, which I think just signifies the higher kill count for the segment.
Holy shit, you just kind of blew my mind. I never even mentally registered those or considered that stuff in the background would contribute to gameplay and your score, by "stuff" I meant notes, turns, etc.. I just tested and that's totally what the kills are, you're right:
The "tentacles" that creep up from the background in certain parts of each level can be killed by performing a ground pound on a note (you don't need a multiplier going). The pulse from your pound will travel along the track and kill the tentacles by making the scream sound and the tentacles will flash red. The number of kills = number of tentacles hit.
I don't know if that's something really to be considered often with high level play since you're going to be constantly pounding to generate health packs anyways, but that's still crazy. This game man. edit: Also my apologies at missing
your post a couple hours ago that referenced it, I got carried away testing stuff.