What couldn't you understand in the above reverse part?It is very much in line with the statement of the Theorem of Divisibility of a Number by 11.
In the reverse, every alternate term has been reversed. That is what the statement states. When you'll test divisibility of any given no, then just apply the reverse part.
Let's take an example, we've got to test the divisibility of 121. So this can be written as
1X1+10X2+100X1. The break up is done.[See reverse logic]. Now in d reverse logic ignoring d coefficients we have, going with the rule of every alternate term negative, 1-2+1, so 2-2=0.
Finally, goes your answer. :)
If still u don't get go thru the comments. :)