The IoC threads work by converting park-able functions into Single-Static Assignment (SSA) form [1] and then compiled to a state machine. Essentially, each time a function is "parked", it returns a value indicating where to resume from. These little state machine functions are basically big switch statements that you don't need to write by hand. This design is inspired by C#'s async compilation strategy. See the EduAsync Series [2] on John Skeet's blog, and the compilation post [3] in particular. Once you have these little state machines, you just need some external code to turn the crank.

[1]: http://en.wikipedia.org/wiki/Static_single_assignment_form

[2]: http://msmvps.com/blogs/jon_skeet/archive/tags/Eduasync/defa...

[3]: http://msmvps.com/blogs/jon_skeet/archive/2011/05/20/eduasyn...