Cooperative scheduling is like reference-counting: you have to tell the compiler when it's safe to context-switch. Preemptive scheduling is like garbage-collection: the runtime decides on its own when it will switch, and you have to deal with making that safe.

Reduction-based scheduling is a hybrid approach, and is, in this analogy, a bit like Objective-C's Automatic Reference Counting. Under reduction-based scheduling, context switches are a possible side-effect of exactly one operation: function calls/returns. This means that it's very easy to reason about when context switches won't happen (so it's not hard to write atomic code), while not having to worry about making them happen yourself (because, in a language that defines looping in terms of recursion, all code-paths have a known-finite sequence of steps before either a function call or a return.)