Fisher test discrimination power of a variable

On the TMVA::MethodFisher page:

[ ... isher.html]( 

it says that

" the Fisher analysis aims at simultaneously maximising the between-class separation, while minimising 
  the within-class dispersion. A useful measure of the discrimination power of a variable is hence given 
  by the diagonal quantity: Bii/Wii/."

Now, I understand that the size (n x n) of the Between (B) and Within-Class (W) matrices are given by the number of input variables, n. Given this, how can Bii/Wii be a “useful measure of the discrimination power” of a single variable? At least two variables are required to construct the matrices B and W, so the respective traces would represent more than one variable.