the switch cast predates spey casting.
Basically the switch cast is exactly the
same as the single-spey, but with one small
difference: the fly does not kiss
the water on the backcast. This small
difference loses many yards on the cast,
however it should also be pointed out that it
also makes for a more delicate cast. So if
you are casting to a fish only a short
distance away then this is the cast to use.
This said, now the Switch Cast is most commonly used to refer to the Jump or Dynamic Roll. Don't you just love us flycasting instructors?