that is the term given to change in acceleration rates
Kind of like the acceleration of the acceleration.
Apart from the physics of valvetrain momentum there would also seem to be the engines ability to develop vacuum given a particular rod to stroke ratio etc. Sometimes it seems amazing the things run at all; parts reciprocating in multitude directions, oil leaking from every joint, different expansion rates and thermal cycles and on the other side of the piston a clean burn that satisfies 500,000 government enviromentalist.
I'm sure Dave Shoe could add some poetic descriptions here......
