Astonishing!
I suppose that the idea is: if she survives she is guilty and must be burned, if she drowns she was innocent.
On a much less dramatical level, there is mention of a medieval tradition in some villages in Brittany (and probably elsewhere as well) “le saut des poissonniers“. After the Carême (Lent religious period when eating meat was forbidden) the fishmongers of the village jumped ...voluntarily... in a nearby pond to make people laugh and be friends again, because they earned a lot of money during the Carême.