Here is my attempt to document them.

All destructive methods can optionally take one or two trailing function arguments, e.g., .find("div", fn, fn2).

If no functions are provided, the chain receives the modified list of nodes changed by the destructive method.

If there is one trailing function arg, it does .each(fn) with the changed set but returns the original list of nodes to the chain.

If there are two trailing function args, the first (fn) is treated as above. However, if the changed set was empty (and thus fn was never called) it does .each(fn2) with the original set and also returns the original set for chaining.

The effect is similar to an if-then-else, and lets you perform operations on sets of elements without disturbing the original elements in the chain.