Douche is a jet of liquid applied to part of the body to clean it. Usually it refers to the vagina or rectum.
It may be important to self-administer a douche to cleanse the anal tract of excrement as a prelude to anal intercourse. Among fistfuckers (both homo- and hetero-sexual), it tends to refer to a high colonic enema that removes nearly everything in the lower gastrointestinal tract.
A basic female douche kit is fine for a low douching; this is basically a bulb syringe. The more sophisticated have attachments to their bathtub shower head that let them switch to a hose, where warm water is introduced into the rectal cavity, then expelled, with shit. (Drain filters should be removed.) High practitioners install plumbing subsystems to their shower head, with red-rubber hoses filtering down to a double-female-ended garden nozzle that can be carefully manipulated to vary how much water pressure is allowed up the butt; this is the insertional hose, a safe, soft outlet. These high-hosers tend to go for the hot-and-stinky release of liquid colonic material. Scat and fisting are related, but for anal-erotics, it is mainly the smell of one's own hot-and-stinky shit, and then what is done with one's butt.