selects on skew_25d's own value against its own trailing rolling median and then measures skew_25d's own subsequent change: the event is chosen because a quantity is unusually far from its own recent typical level, and the same quantity's subsequent change is then measured. An unusually extreme value tends to be followed by a less extreme one purely by chance, so this design finds an apparent effect even in data with no relationship at all. Testing it honestly needs a null that reproduces that tendency, which this harness does not implement. Not tested -- not rejected on evidence.