It is the call stack that is being copied. It doesn't have to do so much with stack allocated variables (which may be created in Java according to the whim of t

It is the call stack that is being copied. It doesn't have to do so much with stack allocated variables (which may be created in Java according to the whim of the compiler) but with capturing the current state of a computation in a relatively light weight manner, so that light weight threads / coroutines can be saved and restored without the context switch and memory overhead of a full thread. This is basically the old "green threads" implementation used in the early JVMs. The big idea behind this is the concept of a continuation, and a conversion between programs in "direct style" (the way you normally write them) and "continuation passing style" which makes control flow explicit. You could start reading here if interested: http://matt.might.net/articles/by-example-continuation-passing-style/ http://matt.might.net/articles/by-example-continuation-passi... Or read about how Scheme compilers work to support first class continuations, and the tricks you can play with them.