How does this differ from scheme-style continuations? IIRC, full continuations in the scheme sense of the term are currently impossible in Java as they require forbidden stack manipulation. Or is what's being proposed here something entirely else?
Oleg Kiselyov has written about it extensively: http://okmij.org/ftp/continuations/
Each time you resume the execution of a fiber, it continues to execute until it reaches its next yield point. There is no way to go "back in time" and resume execution from a previous state of the fiber, which is what multishot coroutines or the fork system call would let you do.
They're changing the JVM and Java specs, so what is forbidden or not will change.