It is when you can divide fractions easier. Here is an example (the slash is just a different sign to show division):
[tex] \frac{2}{5}
[/tex]÷[tex] \frac{5}{6} [/tex]= [tex] \frac{2}{5} [/tex] x [tex] \frac{6}{5} [/tex]= [tex] \frac{12}{25} [/tex]
Sorry if you can not see it... but it is basically Keep (fraction) , Change (division symbol to a multiplication symbol) , Flip (fraction)