Even a stock system usually has left over energy. That's the ringing seen after the spark ends.

Below is a borrowed image so every can see what we're talking about.


You guys who are into this stuff might find this post interesting even though its not the same systems we're discussing. Different coils are being tested to see which one is the overall best for the system and use.
https://bmw.spoolstreet.com/threads/ignition-coil-comparison-primary-energy.1613/