a witches gate is most frequently used by those who are not permitted to transgress boundaries without permission. the effect is that it turns part of a given structure spatially inside out so that part of the inside is now the outside, and so forth. the person can then step into the newly arranged "outside" part of the structure, and cancel the effect, allowing them to technically bypass the restriction and get at delectables that just refuse to come out in response to charm, blandishments, or threats. works even against warded areas, to boot.
oddly enough, it is one of the pieces of mythology that somehow didn't find its way into d&d.