You're supposed to be making tons of small circular strokes and reaching every surface and taking the right amount of time... but it's easy to mess that up subtly. A lot of people make wide strokes for example or are unable to get the large head of the toothbrush in the corners of their teeth or use too much pressure...

That's why I went with the reciprocating electric toothbrush over the vibrating kind. The reciprocating type has a smaller head that can reach more places, and it's less dependent on correct usage than the vibrating kind.

Mine vibrates to let me know roughly how long to spend in each quadrant of my mouth and lights up red if I'm pushing too hard