What it teaches
Choosing between rz and ż. Unlike ó and u, this one has a rule a child can apply on sight: after certain consonants it is always rz — przyjaciel, krzesło, drzewo.
The second useful group is the -arz ending on names of people and trades: lekarz, malarz, piekarz. Between them the two groups cover most of what a child of this age writes.
How it works
A word with a gap and two choices. The words are the ones children of seven and eight actually write, not the ones that make the rule look tidiest.
On a wrong answer the letter before the gap is highlighted, because in most of these words that letter is the entire reason for the answer.
What to look for
Say the consonant before the gap out loud together. If it is one of the nine, the answer is settled without any need to remember the word.