Explain how order clause can be exploited in Rails

Question

I am having difficulty understanding how this section from this website on Rails SQL Injections works.

Taking advantage of SQL injection in ORDER BY clau

Answer 1

If you are trying to determine the value of a field you know is in the table, but not being returned in the select you could iterate over it in the order by, until you get the value:

 ORDER BY CASE WHEN variableIdLikeToDiscover < 'N' then 1 else 0 end

Then see whether it is greater than or less than 'N'. If it's less than, next you could try:

 ORDER BY CASE WHEN variableIdLikeToDiscover < 'F' then 1 else 0 end

And so on and so forth until you have (eventually) determined the value.