Hulda Clark in a youtube video (Identifying parasites in a toilet, part 1) explains that the change in salinity from 1% salt (blood, the fluke's natural environment) to the contents of the colon during a flush, makes the delicate membranes of the critter burst. They kinda shrink to an gooey mess with the egg-strings hanging out because the eggs are meant to naturally disperse if the original parasite dies.
The vid includes several easy-to-see specimens in formaldehyde, and also explains the process of toilet investigation.