The sciences behind teeth brushing

Why brush teeth?

Feynman questioned the utility of tooth brushing. Is the efficacy of tooth brushing a myth? Unlikely. Fluorides in toothpaste (and drinking water) is shown to reduce solubility of enamel, enhance remineralization of enamel and interfere with enzyme activities in oral bacteria. After brushing, saliva show measurable increase in fluoride content. The dissipation of fluoride apparently depends on what you do after brushing, e.g., eating immediately reduces fluoride content in saliva by 12-15 times.

Brushing after meal? Before meal?

There is some evidence suggesting tooth brushing within 30 minutes after meal (especially a meal containing acidic food) leads to increased abrasion of enamel. The theory is that acidic food in a meal soften the enamel and make it vulnerable to mechanical wearing (i.e., brushing). Although the error bar in the aforementioned study is large, it is of our own interest to err on the safe side!

Finally, a study suggests a toothpaste 'massage' once a day superposed on regular brushing boost up fluoride concentration in saliva.

Long, horizontal brushing strokes are bad?

Various sources in internet claims that long horizontal strokes produces more abrasion than vertical ones, and even cause gum recession. However, I failed to find a first-hand, real study on this effect, and to be honest with you, it doesn't make sense to me.