Debating counterplan permutations

Why permute the counterplan?

What makes a good permutation?
-not mutually exclusive
-net beneficial (avoids disad/solves key advantages)

What are the types of permutations?
-intrinsic
-severance

(See also intrinsicness arguments as an answer to disad–there is a quick lecture at the Georgetown Debate Seminar about this–which is available for free online).