Why it works: people run the check that would confirm their hunch. The check that halves the field is almost always a different one, and it feels worse to run because it does not point at anything — it only removes four possibilities. A check that eliminates nothing still costs you. That is not a penalty, it is the arithmetic of a real diagnosis, and a room feels it within two moves.
Make them say what a check will rule out before running it. That one habit is the activity.