The hips are the key Simon, the approximate CofG of the human body is fractionally above the hips.
If you tackle and drive a player above the hips, you can knock them onto their back without lifting them
https://youtu.be/CsJZaYOgFhc
But if you tackle and drive below the hips, you are likely to flip them over
https://youtu.be/oGY83tyX66I?t=11
If you drive, you are moving forward forcefully. If you lift,you are usually more or less static initially.
(Grey areas as usual, of course).