Stops jinxes from working
Wilbert Slinkhard said that the term "counter-jinx" was a misnomer, implying that they were intended with just as much harm as the spells they countered, though Hermione Granger disagreed with this assessment.
Practical Defensive Magic and Its Use Against the Dark Arts contains moving colour illustrations of the various counter-jinxes and hexes that it illustrates.
- Harry Potter and the Order of the Phoenix (mentioned twice, once in Defensive Magical Theory and again by Fudge when Marietta betrays the D.A. to him)