When a Y is encountered, the current thread forks, with the current cell being zeroed in the parent thread, and the pointer being moved one to the right and that cell set to 1 in the child.
Because Brainfork is a proper superset of Brainfuck, all Brainfuck programs that do not contain the letter Y are also valid Brainfork programs with identical semantics.
Because Brainfuck ignores all unknown instructions (such as Y), all Brainfork programs are also valid Brainfuck programs, although they may (of course) behave very differently.
Use the -Y flag.
With the "Y[-<]" up front, this brainfuck example that normally outputs "Just another brainfuck hacker."
Y[-<] +++[>+++++<-]>[>+>+++>+>++>+++++>++<[++<]>---]>->-.[>++>+<<--]>--.--.+. >>>++.<<.<------.+.+++++.>>-.<++++.<--.>>>.<<---.<.-->-.>+.[+++++.---<]>>[.--->]<<.<+.++.++>+++[.<][.]<++.
.. instead outputs "JJusutst a anonoththere r brbraiainnffuucckk hahcackekrer.." If the parent had paused a couple cycles, the execution would have been back in sync and the output would have consisted of unjumbled double characters.
This example spawns a thread, then the new thread and the main thread spawn new threads and so on.