They will need to "clean" their system so the experience is enjoyable and hygienic, reason why they require notice.

You don't want to be poking around in a pile of shit, do you?

As Paloma said, prep usually means "cleaning out," which usually involves some fasting an enemas.

For some girls that may be as little as avoiding curry and taking a dump in advance.  For most it means being very careful about what she eats and drinks , and then doing an enema.

They usually use an enema to clean their anus prior to play!

I provide anal on my clients meaning I bend them over, so to avoid a mess (caca) I will provide an enema to clean their naughty hole before playtime.