What it teaches
Choosing between ó and u in words where the ear cannot help, because in modern Polish the two are pronounced the same.
The words are grouped around the rule that does most of the work: ó appears where the word is related to one with o or e in it — wóz beside wozy, niósł beside niesie. The exceptions have to be learned, and they come back through the review rather than being explained away.
How it works
A word appears with a gap, and two letters to choose from. This is the one subject on the site that is in Polish, for the obvious reason.
A wrong letter is not accepted. The word stays, and the related word that carries the o or the e is shown as the hint — the rule, not the answer.
What to look for
The endings are the reliable part: -ówka, -ówna, -ów take ó almost without exception. If those are solid, the rest is a shorter list than it looks.